Alain Jérôme

Laure, Tao, Corentin, Benjamin Pascale

Maud Rose

Valeska

Antigone Anita Benjamin

Francesco Daizhong

Les études sur l'évolution des galaxies ont été révolutionnées au cours des dernières décennies grce aux nouveau télescopes et aux instruments de pointe. Les interféromètres Herschel, ALMA et NOEMA permettent d'observer l'émission des poussières tracant la formation des étoiles, invisible dans les bandes infrarouges proche observées par les télescopes optiques. Leur grande sensibilité permet la détection des galaxies les plus lointaines, ainsi que des galaxies locales les plus faibles dont la formation et l'évolution remettent en question les théories et les modèles actuels. La technique de l'unité de champ intégrale permet de combiner l'information spatiale et les propriétés extraites des spectres, permettant une étude approfondie de la cinématique des galaxies.

Dans cette thèse, je me concentre sur la formation des étoiles aux grandes échelles des galaxies (≥ kpc). En utilisant les données des téléscopes Herschel, ALMA, NOEMA, et SAMI IFU, ma thèse tente de mettre en lumière les questions suivantes : (1) Comment les premières galaxies ont-elles formé des étoiles à partir du gaz contenant peu de métaux ? (2) Comment les galaxies massives du début de l'Univers ont-elles rapidement constitué leur masse et finalement cessé de former des étoiles ? (3) Quel est le rle joué par les environnements dans lesquels elles résident ? (4) Comment la rétroaction de la formation des étoiles affecte-t-elle la cinématique des galaxies ? Premièrement, je présente l'étude de six galaxies optiquement sombres extraites de l'étude GOODS-ALMA (la plus grande étude cosmologique à 1,1 m) dont le décalage vers le rouge est supérieur à z = 3. Je présente des preuves que quatre d'entre elles appartiennent à la même surdensité de galaxies, situé à z ≥ 3.5. L'une d'entre elles, AGS24 (M ı = 10 11.32 +0.02 ≠0.19 M § ), est la galaxie la plus massive, sans noyau galactique actif et situé à z > 3. Cette dernière se situe également au centre du pic de densité de surface des galaxies, suggèrant que la surdensité environnante est un proto-amas en cours de virialisation et qu'AGS24 est un candidat progéniteur d'un futur amas de galaxie plus brillant.

Deuxièmement, en me basant sur une observation NOEMA, j'ai étudié la teneur en gaz moléculaire dans IZw18, l'une des galaxies les plus pauvres en métal de l'Univers local. J'ai obtenu une limite supérieure d'émission de CO(J=2-1) qui est dix fois plus faible que précédemment déterminée, ceci malgré la forte activité de formation d'étoiles. Sa faible teneur en CO (par rapport à sa luminosité infrarouge), son taux de formation d'étoiles, et sa luminosité [C II] indiquent un changement drastique dans la structure du MIS (environ quelques pourcents de la métallicité solaire). En particulier, la luminosité élevée [C II] par rapport au CO implique un réservoir moléculaire plus important que l'émetteur de CO dans l'IZw18.

Troisièmement, mes travaux sur les sources d'énergie des mouvements turbulents montrent qu'à des échelles inférieures au kpc, les galaxies locales formant des étoiles du relevé SAMI présentent une iii distribution plate de la dispersion de vitesse du gaz ionisé en fonction de la densité de surface du taux de formation d'étoiles. Cependant, le plancher de dispersion de vitesse est plus élevé que ce que prédisent les modèles "feedback-driven". Cela suggère que des sources supplémentaires à la rétroaction de formation d'étoiles entranent des mouvements aléatoires du MIS dans les galaxies formant des étoiles.

Enfin, je présente mes travaux sur l'émission de poussières spatiallement résolue des galaxies extrêmement pauvres en métaux observées par Herschel. Ces galaxies présentent des températures de poussière plus élevées et des indices d'émissivité plus faibles que ceux des galaxies spirales. Environ la moitié de l'émission à 100 µm provient de la poussière chaude (50 K), par opposition à la composante froide (≥20 K) de la poussière. Les couleurs de l'infrarouge lointain sont toutes liées aux densités de surface des jeunes étoiles, mais pas aux densités de surface des masses stellaires. Cela suggère que leur poussière (70 ≥ 350 µm) est principalement chauffée par le rayonnement des jeunes étoiles.

iv

Acknowledgement

It came as a surprise when I got to know that I could also do a joint-PhD at Université de Paris Saclay and it was already such a great pleasure to come to CEA as a visiting student for two years, thanks to everyone at CEA as a host and the financial support from China Scholar Council.

During the two years in France, I have gained a lot of knowledge not only from academia but also from everyone I met.

First of all, thank you, David. Thanks for hosting me at CEA and for mentoring me in this thesis. Your passion for science inspires me. It reminds me that being a researcher is not only a way to make a living, but more essentially a lifestyle. A lifestyle to stay curious about everything, to observe what is happening around us, to crack down what is behind what we see. It reminds me of how I was originally fascinated about science: to think, to ask questions originated from my daily life and finally to describe, to explain, and to predict these phenomena. I am grateful for your patience with me, for explaining to me the big pictures and for so much help with my life in France.

Thanks Emanuele for always taking so much time for the Journal Club and for the valuable discussions, your door is always open when I need help. Thanks also to many other people,

Introduction

" ✏ˆ ∆ When a kid I didn't cognize the moon,

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-⌦‰⌫ L↵ Ballad of Ancient Bright Moon N} Bai Li

1.1 Star formation: the driver of galaxy evolution

Galaxies are made of stars, dust, gas and dark matters. They are gravitationally bound systems showing various properties in morphologies, luminosities, colors, sizes and masses, etc. Such diversities suggest complicated processes in galaxy evolution and formation (e.g., [START_REF] Mo | Galaxy Formation and Evolution[END_REF]. Nowadays, most studies converge to a scenario where galaxies grow mainly through a secular process (e.g., [START_REF] Schreiber | The Herschel view of the dominant mode of galaxy growth from z = 4 to the present day[END_REF]: Gas accretes from the intergalactic medium (IGM), then it cools, condenses into molecular clouds and finally collapses into stars, hence, fuels star formation [START_REF] White | Core condensation in heavy halos: a two-stage theory for galaxy formation and clustering[END_REF]. At the late stage of their lives, stars release part of their masses back to the interstellar medium (ISM) through stellar winds, planetary nebulae and supernovae. The released gas and dust, as well as energy, are involved in the formation of the next generation of stars(e.g., [START_REF] Larson | Effects of supernovae on the early evolution of galaxies[END_REF][START_REF] Martin | Properties of Galactic Outflows: Measurements of the Feedback from Star Formation[END_REF]. Therefore, star formation is considered as the key process driving galaxy evolution.

However, the mechanisms that regulate each step mentioned above are complex: What controls the gas inflow from the surrounding environment of galaxies and stars? How efficient is the conversion of the available gas reservoir into stars? Whether the gas ejection and heating by star formation feedback accelerate or hinder further star formation?... In addition, supermassive black holes at the center of galaxies may undergo a temporarily active phase of active galactic nucleus (AGN). The efficient gas accretion triggers AGNs which then release a significant amount of energy into the host galaxies [START_REF] Kormendy | Coevolution (Or Not) of Supermassive Black Holes and Host Galaxies[END_REF][START_REF] Heckman | The Coevolution of Galaxies and Supermassive Black Holes: Insights from Surveys of the Contemporary Universe[END_REF]. This further complicates galaxy ecosystems.

Galaxies are not isolated systems. When we place them in a broader context, they pinpoint the most luminous and the most massive spots of matter distributions in the Universe. Such wide distributions are called large-scale structures, and are always in the form of web-like networks, hence also called cosmic web [START_REF] Peacock | A measurement of the cosmological mass density from clustering in the 2dF Galaxy Redshift Survey[END_REF]. Large-scale structures provide passages for gas to accrete into galaxies and to replenish the fuel of star formation [START_REF] Dekel | Cold streams in early massive hot haloes as the main mode of galaxy formation[END_REF]. Some galaxies would gather in the knots of cosmic webs, therefore they are more likely to interact with each other. The interactions can be violent, such as mergers, or mild, such as fly-bys. In either case, the distribution and physical states of the stars, ISM and halos in the galaxies can be dramatically altered after the interactions and the evolution paths of the galaxies will be significantly changed since then.

The extragalactic astronomy is a relatively young field of research. It was not until a century ago that, Hubble measured the distances of the Cepheid variable stars in Andromeda and confirmed that there are systems external to our Milky Way [START_REF] Hubble | Extragalactic nebulae[END_REF]. The study of galaxy formation and evolution relies heavily on observations. Compared to gravitational waves originating from extremely rare events of merging binary systems, electromagnetic emission from galaxies, mainly consist of direct or reprocessed starlight, are ubiquitous and relatively easier to obtain. Thanks to the rapid development of the modern technology, especially the tremendous progress in manufacturing powerful telescopes, exciting breakthroughs have been made through astrophysics observations, for example, last year, astrophysicists obtained the first direct confirmation of the existence of black holes in M83 (Event Horizon Telescope Collaboration et al., 2019), using the Event Horizon Telescope, a planet-scale array of ground-based radio telescopes. Yet, there is more to explore.

So far in extragalactic astronomy, despite the broad diversities of galaxies, extensive observations have revealed tight scaling relations between galaxy properties, such as the star formation law (star formation rate surface density vs. gas surface density), the galaxy main sequence (star formation rate vs. stellar mass), which will be introduced in detail below, as well as the stellar mass-metallicity relation, the Tully-Fisher relation (total mass vs. rotational velocity) etc. The comparison between these relations with models can better constrain the processes regulating the star formation activity in galaxies to further understand how galaxies in the early Universe evolved to the ones on the present day.

In this thesis I have worked on different topics on star formation in galaxies. In the following, I am going to specify the main topics I have been working on during my thesis on star formation in galaxies.

Star formation obscured by interstellar dust

UV emission of galaxies is dominated by the most massive stars (OB stars), which end their lives shortly after their birth (< 100Myr), compared to the vast majority of other stars (e.g., it is estimated to be 10 Gyr for the Sun). Therefore, the existence of OB stars, as indicated by their UV emission, suggests a recent star formation within a time scale of ≥100 Myr. Then UV luminosity is an idea tracer of the star formation rate (SFR) of galaxies (Kennicutt, 1998a). However, as mentioned above, stars produce heavy elements, also known as metals, and release them into the ISM at the late stage of their evolution. A significant fraction of metals are locked into the interstellar dust. This suggests that star formation is always accompanied by dust formation. UV photons are energetic, with wavelengths between 150 and 300 nm, smaller than the typical size of dusts. As UV photons emitted by a massive galaxy travel through the galaxy, they can be easily absorbed or scattered by dust. Hence SFR estimated from UV can be heavily underestimated. Nevertheless, the absorbed energy will be re-emitted by dust in the far-infrared (FIR) through thermal radiation. An example is shown in Fig. 1.1-b. By observing in the FIR1 , astrophysicists can recover star formation hidden by the interstellar dust.

Fig. 1.1: a: Spectral energy distributions (SEDs) for typical galaxies -an old elliptical galaxy (red), two types of spiral galaxies (Sb in green and Sd in blue), an AGN (Markarian 231, solid black), a QSO (dotted black), and a merging and star-bursting galaxy Arp 220. Template spectra are taken from [START_REF] Polletta | Spectral Energy Distributions of Hard X-Ray Selected Active Galactic Nuclei in the XMM-Newton Medium Deep Survey[END_REF]. Courtesy: CANDELS blog. b The spectral energy distributions (SEDs) of a typical star-forming galaxy (H-ATLAS J090713.1-000322). The total SED is in grey, with the stellar component in red and the corresponding dust emission in blue. The green curve denotes the unattenuated stellar light. Figure from [START_REF] Smith | Herschel-ATLAS: multi-wavelength SEDs and physical properties of 250 µm selected galaxies at z < 0.5[END_REF]. c: Example of SED fitting -an old, dust free galaxy (blue) and a young, dusty galaxy (red) both fit the observed data points (green) in the UV to mid-infrared (MIR) regime. Courtesy: CANDELS blog.

Multi-wavelength study of galaxies

Based on morphologies, Edwin Hubble proposed a classification scheme dividing galaxies into spirals, lenticulars and ellipticals [START_REF] Hubble | Extragalactic nebulae[END_REF]. This is the so-called Hubble tuning fork or Hubble sequence.

Later on, as more photometric filters are available to observe galaxies, a bimodal distribution of red and blue2 galaxies was discovered by the Sloan Digital Sky Survey (SDSS), see Fig. 1.3-Left as an example [START_REF] Strateva | Color Separation of Galaxy Types in the Sloan Digital Sky Survey Imaging Data[END_REF][START_REF] Bell | Nearly 5000 Distant Early-Type Galaxies in COMBO-17: A Red Sequence and Its Evolution since z ~1[END_REF][START_REF] Baldry | Quantifying the Bimodal Color-Magnitude Distribution of Galaxies[END_REF]. This bimodality in color was later found to be related to their morphologies, with galaxies falling in the red sequence mostly elliptical and galaxies falling in the blue cloud mostly spiral 3 . The colors of galaxies is a natural result of their spectral energy distributions (SEDs).

An SED shows the energy distribution of a galaxy across the electromagnetic spectrum, where physical processes occur to the stars, gas, dust and possibly AGNs in galaxies leave their imprints. In Fig. 1.1-a, we show the SEDs of typical galaxies: an old elliptical galaxy, two types of spiral galaxies, an AGN, a QSO, and a merging and star-bursting galaxy Arp220. They show distinct features in their SEDs. Ellipticals show fainter emission in the UV-optical and FIR compared to spirals, because they have few young stars and little dust to emit at these two regimes. In mergers, the intensive star formation produce an enormous amount of young stars and dust that contribute to the bright UV-optical emission However, taking advantage of the prominent features in the UV to MIR spectrum, the colors can be good indicators to classify galaxies. On the other hand, according to Hubble-Lemaître law [START_REF] Hubble | The Velocity-Distance Relation among Extra-Galactic Nebulae[END_REF][START_REF] Lemaître | Expansion of the universe, A homogeneous universe of constant mass and increasing radius accounting for the radial velocity of extra-galactic nebulae[END_REF], the Universe is expanding, and the more distant the galaxy is, the faster it is receding from us. As a result, the SEDs of distant galaxies are redshifted. [START_REF] Steidel | Spectroscopic Confirmation of a Population of Normal Star-forming Galaxies at Redshifts Z > 3[END_REF] identified star forming galaxies at z ≥ 3 with U n GR s rest-frame UV colors. They used a "drop-out" technique: the Lyman break4 , at ⁄ ≥ 912 Å, is redshifted to ≥ 3600 Å at z ≥ 3. When a star-forming galaxy at z ≥ 3 is imaged with a series of filters, e.g., as in Fig. 1.2, it is invisible in the image taken with the filter(s) covering ⁄ AE 3600 Å, or it is "dropped-out".

In addition, the roughly flat spectrum (F ‹ Ã ‹ 0 ) in the far-UV, where emission is dominated by massive stars, is the other feature they used. Star-forming galaxies selected with this technique are called Lyman break galaxies (LBG) 4 . They are biased to UV bright galaxies with low dust content, because dust would absorb the photons emitting at rest frame wavelengths longer than 912 Å, hence the Lyman break would be washed out. BzK colors (observed frame) proposed by [START_REF] Daddi | A New Photometric Technique for the Joint Selection of Star-forming and Passive Galaxies at 1.4 < ~z < ~2.5[END_REF], making use of the CIV absorption line at 1550 Å (rest-frame, same below), the breaks and absorption features at 2200 -3000 Å, and the 4000 Å break 5 , can effectively select both star-forming galaxies and passive galaxies at 1.4 . z . 2.5. In order to include dusty galaxies, which tend to be massive so that dust content is bound in their potential well, into the selection, rest-frame infrared colors are needed. [START_REF] Williams | Detection of Quiescent Galaxies in a Bicolor Sequence from Z = 0-2[END_REF] proposed a UVJ selection based on the strong bimodal behaviour found between quiescent galaxies and dusty star-forming galaxies in the rest-frame U -V versus V -J space. The introduced infrared band, J, in V -J, is used to break the age-attenuation degeneracy while the U -V mimics the traditional color-magnitude diagram [START_REF] Strateva | Color Separation of Galaxy Types in the Sloan Digital Sky Survey Imaging Data[END_REF][START_REF] Bell | Nearly 5000 Distant Early-Type Galaxies in COMBO-17: A Red Sequence and Its Evolution since z ~1[END_REF][START_REF] Baldry | Quantifying the Bimodal Color-Magnitude Distribution of Galaxies[END_REF]. Pushing forward to higher redshifts, at z > 3, [START_REF] Wang | Infrared Color Selection of Massive Galaxies at z 3[END_REF] introduced the observed MIR Infrared Array Camera(IRAC) band at 4.5 µm, in the H -[4.5] (observed frame) color to select massive galaxies which have been largely missed by previous methods. The H band traces the 4000 Å break at z > 3 while [4.5] is the redshifted J as used in the UVJ (rest-frame) color selection by [START_REF] Williams | Detection of Quiescent Galaxies in a Bicolor Sequence from Z = 0-2[END_REF]. An additional J-H (observed frame) color is used to rule out the contamination from extremely dusty galaxies at z < 3.

With all these effective methods, a large amount of star-forming galaxies with great diversities (redshift, mass, dust content, etc.) have been accumulated to allow a systematic study of the star formation in galaxies over the cosmic history.

1.2.2 The main sequence of star-forming galaxies

The study of the present and past star formation in individual galaxies relies on detailed studies of the stellar populations within galaxies. Such studies are only feasible with galaxies in the Local Group (within 1.5 Mpc from us) due to observation limitations. However, we can study the dominant drivers of the changes in star formation of galaxies statistically.

Although galaxies show a broad diversity, many tight scaling relations have been found. Hubble tried to interpret the Hubble sequence as a galaxy evolution sequence where ellipticals formed first and spirals later6 [START_REF] Hubble | Extragalactic nebulae[END_REF]. With the advent of large surveys like SDSS, as mentioned previously, the galaxy color-magnitude diagram shows a distinctive bimodal distribution (Fig. 1. . The u-r color is a proxy of specific star formation rates (sSFRs © SFR/M star ) and the magnitude can be converted into stellar masses. Therefore, the distribution of red and blue galaxies in the color-magnitude diagram indicates that stellar masses are regulating the star formation in galaxies. Indeed, star-forming galaxies show a tight correlation ( with a dispersion less than 0.3 dex, [START_REF] Sargent | The Contribution of Starbursts and Normal Galaxies to Infrared Luminosity Functions at z < 2[END_REF]; [START_REF] Schreiber | The Herschel view of the dominant mode of galaxy growth from z = 4 to the present day[END_REF]) between their SFR and stellar masses over a large redshift range, from z ≥ 0 to z & 4, and over a broad stellar mass range from down to 10 9.0 M § to up to 10 11.4 M § [START_REF] Brinchmann | The physical properties of star-forming galaxies in the low-redshift Universe[END_REF][START_REF] Elbaz | The reversal of the star formation-density relation in the distant universe[END_REF][START_REF] Daddi | Multiwavelength Study of Massive Galaxies at z ~2. I. Star Formation and Galaxy Growth[END_REF][START_REF] Noeske | Star Formation in AEGIS Field Galaxies since z=1.1: The Dominance of Gradually Declining Star Formation, and the Main Sequence of Star-forming Galaxies[END_REF][START_REF] Rodighiero | The Lesser Role of Starbursts in Star Formation at z = 2[END_REF][START_REF] Whitaker | The Star Formation Mass Sequence Out to z = 2.5[END_REF][START_REF] Daddi | Two Bright Submillimeter Galaxies in a z = 4.05 Protocluster in Goods-North, and Accurate Radio-Infrared Photometric Redshifts[END_REF][START_REF] Pannella | Star Formation and Dust Obscuration at z ¥ 2: Galaxies at the Dawn of Downsizing[END_REF][START_REF] Magdis | A Multi-wavelength View of the Star Formation Activity at z ~3[END_REF][START_REF] Heinis | HERMES: unveiling obscured star formation -the farinfrared luminosity function of ultraviolet-selected galaxies at z ≥ 1.5[END_REF][START_REF] Pannella | GOODS-Herschel: Star Formation, Dust Attenuation, and the FIR-radio Correlation on the Main Sequence of Star-forming Galaxies up to z 4[END_REF][START_REF] Steinhardt | Star Formation at 4 < z < 6 from the Spitzer Large Area Survey with Hyper-Suprime-Cam (SPLASH)[END_REF][START_REF] Bouwens | UV-continuum Slopes at z ~4-7 from the HUDF09+ERS+CANDELS Observations: Discovery of a Well-defined UV Color-Magnitude Relationship for z >= 4 Star-forming Galaxies[END_REF][START_REF] Stark | The Evolutionary History of Lyman Break Galaxies Between Redshift 4 and 6: Observing Successive Generations of Massive Galaxies in Formation[END_REF][START_REF] Stark | Keck Spectroscopy of 3 < z < 7 Faint Lyman Break Galaxies: The Importance of Nebular Emission in Understanding the Specific Star Formation Rate and Stellar Mass Density[END_REF]. This is the so-called "main sequence" correlation. A minority of galaxies ("starbursts", only a few percent of the total star-forming galaxies, [START_REF] Rodighiero | The Lesser Role of Starbursts in Star Formation at z = 2[END_REF]; [START_REF] Sargent | The Contribution of Starbursts and Normal Galaxies to Infrared Luminosity Functions at z < 2[END_REF]; [START_REF] Schreiber | The Herschel view of the dominant mode of galaxy growth from z = 4 to the present day[END_REF], see also Fig. 1.4-Right) have highly elevated SFRs above the main sequence [START_REF] Elbaz | GOODS-Herschel: an infrared main sequence for star-forming galaxies[END_REF] and there are also galaxies with deeply reduced SFRs below the main sequence. This is illustrated in Fig. 1.3-Right.

The main sequence evolves with redshift, as shown in Fig. 1.4-Left, which implies that at a specific stellar mass, a galaxy would form stars more rapidly at higher redshift, i.e., the sSFR evolves continuously with redshift. In another word, the SFR of a main sequence galaxy at high redshift would be associated to a starburst galaxy with the same stellar mass but at lower redshift. On the other hand, the dispersion of the main sequence barely evolves with redshift, which means that at a specific stellar mass and a specific redshift, ≥ 68% of star-forming galaxies form stars at a rate within a factor of ≥ 2 [START_REF] Schreiber | The Herschel view of the dominant mode of galaxy growth from z = 4 to the present day[END_REF]. If we define the galaxies on the main sequence as "normal" star-forming galaxies, then the normal and starburst galaxies represent the two main modes that control the growth of galaxies, because starbursts are not simply an extension of main-sequence galaxies, as shown by the double Gaussian decomposition of the starburstiness (R SB = SFR/SFR MS ) distribution of star-forming galaxies in Fig. 1.3-Right. In fact, these two populations show different evolution paths. Starbursts are often interpreted as driven by mergers which can substantially boost SFR instantaneously, while normal galaxies form stars more steadily. The low percentage of starburst in star-forming galaxies indicates that galaxy mergers have very limited contribution to the global star formation history. Moreover, at 0 < z < 3, the fraction of starbursts is not evolving with redshift, which means that the "normal" mode dominates the star formation in most galaxies [START_REF] Rodighiero | The Lesser Role of Starbursts in Star Formation at z = 2[END_REF][START_REF] Sargent | The Contribution of Starbursts and Normal Galaxies to Infrared Luminosity Functions at z < 2[END_REF][START_REF] Schreiber | The Herschel view of the dominant mode of galaxy growth from z = 4 to the present day[END_REF]. The steadily evolving main sequence has been used to constrain the cosmic star formation histories.

Fig. 1.4: Left: The evolution of star formation main sequence with redshift. The average SFR of star-forming galaxies as a function of redshift and stellar mass are parameterized as in Eq. 1.1, see also [START_REF] Schreiber | The Herschel view of the dominant mode of galaxy growth from z = 4 to the present day[END_REF]. Right: Double Gaussian decomposition of the distribution of starburstiness (R SB = SFR/SFR MS ). Figures from [START_REF] Schreiber | The Herschel view of the dominant mode of galaxy growth from z = 4 to the present day[END_REF].

The property of a galaxy is reflected on its position relative to the main sequence. For example, galaxies along the main sequence form stars at roughly constant efficiency (SFE © SFR/M gas ) (Daddi et al., 2010a;[START_REF] Tacconi | Phibss: Molecular Gas Content and Scaling Relations in z ~1-3 Massive, Main-sequence Star-forming Galaxies[END_REF][START_REF] Genzel | Combined CO and Dust Scaling Relations of Depletion Time and Molecular Gas Fractions with Cosmic Time, Specific Star-formation Rate, and Stellar Mass[END_REF], which means that SFE is not controlled by the stellar mass of a galaxy. However, SFE increases as galaxies move above the main sequence, and stars form more efficiently in galaxies at higher redshifts (see Tacconi et al., 2018, and references therein). Dust properties show similar behaviours. The dust temperature, T dust , of galaxies, is also roughly constant along the main sequence [START_REF] Magdis | The Evolving Interstellar Medium of Star-forming Galaxies since z = 2 as Probed by Their Infrared Spectral Energy Distributions[END_REF][START_REF] Béthermin | Evolution of the dust emission of massive galaxies up to z = 4 and constraints on their dominant mode of star formation[END_REF][START_REF] Schreiber | Observational evidence of a slow downfall of star formation efficiency in massive galaxies during the past 10 Gyr[END_REF], but it gets higher at higher redshifts [START_REF] Magdis | The Evolving Interstellar Medium of Star-forming Galaxies since z = 2 as Probed by Their Infrared Spectral Energy Distributions[END_REF][START_REF] Magnelli | The evolution of the dust temperatures of galaxies in the SFR-Mú plane up to z ≥ 2[END_REF][START_REF] Béthermin | Evolution of the dust emission of massive galaxies up to z = 4 and constraints on their dominant mode of star formation[END_REF][START_REF] Schreiber | Dust temperature and mid-to-total infrared color distributions for star-forming galaxies at 0 z 4[END_REF] and at higher R SB [START_REF] Elbaz | GOODS-Herschel: an infrared main sequence for star-forming galaxies[END_REF][START_REF] Magdis | The Evolving Interstellar Medium of Star-forming Galaxies since z = 2 as Probed by Their Infrared Spectral Energy Distributions[END_REF][START_REF] Magnelli | The evolution of the dust temperatures of galaxies in the SFR-Mú plane up to z ≥ 2[END_REF][START_REF] Béthermin | Evolution of the dust emission of massive galaxies up to z = 4 and constraints on their dominant mode of star formation[END_REF][START_REF] Schreiber | Dust temperature and mid-to-total infrared color distributions for star-forming galaxies at 0 z 4[END_REF]. In the aspect of morphologies, starbursts are likely to be more compact, as indicated by the emission from the polycyclic aromatic hydrocarbon (PAH) molecules in galaxies. PAH emits in photo-dissociation regions (PDRs), which is always on the surface of molecular clouds while the FIR luminosity depends on the volume of molecular clouds. Hence, the PAH deficit found in starbursts [START_REF] Díaz-Santos | Explaining the [C II]157.7 µm Deficit in Luminous Infrared Galaxies-First Results from a Herschel/PACS Study of the GOALS Sample[END_REF] indicates a shrink in the surface of molecular clouds, where star formation takes place in galaxies. The IR8 © L IR /L 87 proposed by [START_REF] Elbaz | GOODS-Herschel: an infrared main sequence for star-forming galaxies[END_REF] quantify the compactness of the starburst galaxies. The high SFE and compact geometry are also a natural result of galaxy mergers where the ISM are dramatically compressed, leading to enhanced SFE and morphologies are largely disturbed, resulting in a compact core at the interfaces. This again supports the association of starbursts and mergers. Although minor, starbursts may play an important role in the transition from star-forming galaxies to quiescent galaxies.

In my thesis, I adopt the parameterized main sequence as quantified in Eq (9) in [START_REF] Schreiber | The Herschel view of the dominant mode of galaxy growth from z = 4 to the present day[END_REF]:

log 10 (SFR MS [M § /yr]) = m ≠ m 0 + a 0 r ≠ a 1 [max(0, m ≠ m 1 ≠ a 2 r] 2 (1.1)
where r © log 10 (1+z), m © log 10 (M ú / 10 9 M § ), m 0 = 0.5 ± 0.07, a 0 = 1.5 ± 0.15, a 1 = 0.3 ± 0.08, m 1 = 0.36 ± 0.3 and a 2 = 2.5 ± 0.6.

Cosmic star formation history

As the lifetime of a human being is way shorter than the one of a galaxy, it is not possible to see how individual galaxies evolve directly with our own eyes. However, if we switch to a broader point of view, we can see the collective evolution of galaxies by looking deeper into the Universe, which is, observing the galaxies that are more distant. This is feasible because the speed of light is finite. When we look at galaxies from far away, we receive photons that were emitted from galaxies long time ago. Moreover, the universe is homogeneous and isotropic in space and time. It does not matter which direction we are looking from, or at which place we are observing. As long as we know the distance of the galaxies, we can identify we are observing galaxies from how long ago, this is the so-called lookback-time approach.

Owing to the large surveys with space telescopes like Hubble (HST), Spitzer, etc. and ground-based telescopes like the Visible and Infrared Survey Telescope for Astronomy (VISTA), the United Kingdom Infra-Red Telescope (UKIRT), etc., as well as the follow-up observations with 8-m-class telescopes like Very Large Telescope (VLT), Keck, Gemini, etc. to confirm the spectroscopic redshifts, galaxies with redshifts as large as z = 11.1, only around 400 Myr after the Big Bang [START_REF] Oesch | A Remarkably Luminous Galaxy at z=11.1 Measured with Hubble Space Telescope Grism Spectroscopy[END_REF], have been identified. On the other hand, multi-wavelength observations from UV with, e.g., the Galaxy Evolutionary Explorer (GALEX) to FIR with, e.g., Herschel Space Observatory enable techniques like SED fitting to derive the properties of galaxies systematically. With abundant data in hand, astrophysicists are able to construct a picture of cosmic star formation history (Fig. 1.5, see review by [START_REF] Madau | Cosmic Star-Formation History[END_REF].

In Fig. 1.5-ab, it is clear that a large fraction of the emission from star formation is obscured by dust and re-emitted in the IR. The emission at UV only accounts for 10% to 40% of the total star formation rate density (SFRD). The overall dust extinction of galaxies varies with time, with the maximum occurring at around z = 1. Combining the observations at UV and IR, SFR can be calculated with high precision with dust attenuation taken into account properly. As shown by Madau and Dickinson (2014, Fig.1.5-d), the star formation rate density peaked at z ≥ 2, around 3.5 Gyr after the Big Bang, the so-called cosmic noon, then dropped exponentially after z < 1. Cosmic star formation at z ≥ 2 is nine times more intensive than it is today. From the profile of the cosmic SFRD, we can calculate that the majority of the stellar mass today was formed at cosmic noon, i.e., 1 < z < 3. However, we need to keep in mind that the cosmic SFRD by [START_REF] Madau | Cosmic Star-Formation History[END_REF] at z > 3 is poorly constrained because IR data are missing and UV data are limited to the most luminous galaxies (e.g., LBGs) that are not representative (Fig. 1.5-c).

1.2.4 Observing star-forming galaxies in the (sub)millimeter

Observational obstacles in the FIR

As mentioned previously, the FIR emission is important to fully understand the star formation activities of galaxies. However, there are two main obstacles to measure the FIR luminosity. One is the poor atmospheric transmission in the FIR. That's why the telescopes observing in the FIR are mostly space telescopes, i.e., IRAS, ISO, Spitzer, Herschel. And the other is that the angular resolution is worse in the FIR than at shorter wavelength as is defined by the diffraction limit of telescopes:

◊ = 1.22 ⁄ D ,
where ⁄ is the observing wavelength, D is the diameter of the telescope or the baseline of the interferometer. These two obstacles are resolved when we look further. The electromagnetic emission from distant galaxies is shifted to longer wavelengths, then the FIR emission is redshifted to submillimeter wavelength, hence observable on the ground by, for example, Submillimeter Common User Bolometric Array (SCUBA). Furthermore, interferometry technique has tremendously improved the angular resolution with the long baselines, which is the case for submillimeter Array (SMA), NOrthern Extended Millimeter Array (NOEMA) and Atacama Large Millimeter/submillimeter Array (ALMA), etc..

K-correction

A K-correction is applied to convert the observed-frame flux to the rest-frame flux. It depends on the shape of SEDs. For star-forming galaxies, as already shown in Fig. 1.1, the dust emission approximates

Galaxies detected at submillimeter

The observations of the extragalactic background light (EBL) has revealed that that the cosmic infrared background (CIB) has the same brightness as the cosmic optical background (COB) (Fig. 1.7), implying that the amount of starlight absorbed and re-radiated by dust is comparable to the one directly from stars (see [START_REF] Cooray | Extragalactic background light measurements and applications[END_REF], for a review). The CIB can be resolved into bright sources by the submillimeter telescopes. It has been also found that star formation is more attenuated at high redshift [START_REF] Hauser | The COBE Diffuse Infrared Background Experiment Search for the Cosmic Infrared Background. I. Limits and Detections[END_REF]. Especially at the cosmic noon, dust reprocessed emission dominates the star formation emission, as much as one magnitude higher than direct starlight (Le Floc'h et al., 2005;[START_REF] Rodighiero | Mid-and far-infrared luminosity functions and galaxy evolution from multiwavelength Spitzer observations up to z ~2.5[END_REF][START_REF] Gruppioni | The Herschel PEP/HerMES luminosity function -I. Probing the evolution of PACS selected Galaxies to z 4[END_REF][START_REF] Magnelli | The deepest Herschel-PACS far-infrared survey: number counts and infrared luminosity functions from combined PEP/GOODS-H observations[END_REF], as is also shown by Fig. 1.5-a. Therefore, the understanding of dust-obscured star formation at cosmic noon relies on the observation at submillimeter wavelength. Since their discovery, submillimeter-selected galaxies (SMGs, S 850µm > 5 mJy, see [START_REF] Blain | Submillimeter galaxies[END_REF], for a review) detected by single-dish submillimeter telescopes have unveiled a population of massive galaxies undergoing extremely intensive star formation accompanied with a huge amount of dust. This means that SMGs could accumulate a huge amount of stellar mass rapidly. Hence, they have been expected to be ideal progenitors of massive, passive and compact galaxies found at z = 1 ≥ 2 and ultimately local elliptical galaxies [START_REF] Cimatti | GMASS ultradeep spectroscopy of galaxies at z ~2. II. Superdense passive galaxies: how did they form and evolve?[END_REF][START_REF] Whitaker | The Star Formation Mass Sequence Out to z = 2.5[END_REF][START_REF] Toft | Submillimeter Galaxies as Progenitors of Compact Quiescent Galaxies[END_REF][START_REF] Simpson | An ALMA Survey of Submillimeter Galaxies in the Extended Chandra Deep Field South: The Redshift Distribution and Evolution of Submillimeter Galaxies[END_REF][START_REF] Valentino | Quiescent Galaxies 1.5 Billion Years after the Big Bang and Their Progenitors[END_REF]. Because of their high masses, SMGs have deep gravitational potential wells to sustain a huge amount of dust against outflows, and the heavy dust obscuration makes them extremely faint in the rest-frame UV-optical wavelength. That's why there were not discovered until the advent of submillimeter telescopes and that's why submillimeter telescopes are the most power tool to search for such massive objects in the early Universe. However, due to the poor angular resolution of single dishes, as mentioned previously, one detection at the resolution of more than 15 ÕÕ may turn out to be multiple sources blended in the same beam. The ALMA interferometer array has brought unprecedented improvement to the sensitivity and spatial resolution of submillimeter observations and enables precise counterpart identifications of the SMGs and further study on their properties.

The definition of SMGs is based on the submillimeter flux, hence it is not physical. As a result, SMGs are not a homogeneous population. They show a wide variety of properties indicated by their multiwavelength observations (see reviews by [START_REF] Casey | Dusty star-forming galaxies at high redshift[END_REF][START_REF] Hodge | High-redshift star formation in the ALMA era[END_REF]. Especially, when looking at the relation between their stellar masses and SFRs, some SMGs are on the main sequence, and some fall in the starburst regime, (Fig. 1.8-Left, e.g., da Cunha et al., 2015;[START_REF] Danielson | An ALMA Survey of Submillimeter Galaxies in the Extended Chandra Deep Field South: Spectroscopic Redshifts[END_REF][START_REF] Lang | Revealing the Stellar Mass and Dust Distributions of Submillimeter Galaxies at Redshift 2[END_REF][START_REF] Franco | GOODS-ALMA: The slow downfall of star-formation in z = 2-3 massive galaxies[END_REF]. In general, most SMGs show more compact dust distribution compared to their stellar component (Fig. 1.8-Right, e.g., [START_REF] Elbaz | Starbursts in and out of the star-formation main sequence[END_REF][START_REF] Hodge | ALMA Reveals Potential Evidence for Spiral Arms, Bars, and Rings in High-redshift Submillimeter Galaxies[END_REF][START_REF] Puglisi | The Main Sequence at z 1.3 Contains a Sizable Fraction of Galaxies with Compact Star Formation Sizes: A New Population of Early Post-starbursts?[END_REF][START_REF] Lang | Revealing the Stellar Mass and Dust Distributions of Submillimeter Galaxies at Redshift 2[END_REF], which supports the evolution path where SMGs are progenitors of the compact quiescent galaxies at z = 1 -2, while some studies found extended [START_REF] Rujopakarn | VLA and ALMA Imaging of Intense Galaxy-wide Star Formation in z 2 Galaxies[END_REF][START_REF] Cibinel | ALMA constraints on star-forming gas in a prototypical z = 1.5 clumpy galaxy: the dearth of CO(5-4) emission from UV-bright clumps[END_REF]. A statistic way to investigate the evolution of SMGs is to study their number counts [START_REF] Blain | Submillimeter galaxies[END_REF][START_REF] Stach | An ALMA Survey of the SCUBA-2 Cosmology Legacy Survey UKIDSS/UDS Field: Number Counts of Submillimeter Galaxies[END_REF][START_REF] González-López | The ALMA Spectroscopic Survey in the HUDF: Deep 1.2 mm Continuum Number Counts[END_REF][START_REF] Popping | The ALMA Spectroscopic Survey in the HUDF: A Model to Explain Observed 1.1 and 0.85 mm Dust Continuum Number Counts[END_REF]. The significant normalization of the number counts challenges the current theoretical models to explain how these large number of massive galaxies formed in the early Universe (see [START_REF] Casey | Dusty star-forming galaxies at high redshift[END_REF], for a review).

The GOODS-ALMA survey

The SMGs studied by ALMA have been the follow-ups of previously detected sources by single dish submillimeter telescopes and color-and mass-selected galaxies from deep surveys at optical/nearinfrared wavelength. This will lead to a bias towards some specific population. In addition, the enormous dust content indicated by the bright submillimeter emission hinders some SMGs, perhaps the most massive ones, from emitting in the optical/NIR, hence missed from pre-selections. A homogeneous blind survey is needed to obtain a more general population.

The GOODS-ALMA survey (PI: D. [START_REF] Franco | GOODS-ALMA: 1.1 mm galaxy survey. I. Source catalog and optically dark galaxies[END_REF][START_REF] Franco | GOODS-ALMA: Using IRAC and VLA to probe fainter millimeter galaxies[END_REF], covers the largest contiguous area of 69 arcmin 2 in the GOODS-South field by ALMA at 1.1 mm. The original image has a 0.24 ÕÕ beam and mean depth of 110 µJy beam ≠1 . It was then tapered to 0.60 ÕÕ to reduce the number of independent beams, hence the number of statistical spurious detections. The rms sensitivity is then 182 µJy beam ≠1 . A total of 35 galaxies have been detected above 3.5 ‡ in GOODS-ALMA [START_REF] Franco | GOODS-ALMA: Using IRAC and VLA to probe fainter millimeter galaxies[END_REF]. These include 19 galaxies detected above the 4.8 ‡ limit from a blind detection approach [START_REF] Franco | GOODS-ALMA: 1.1 mm galaxy survey. I. Source catalog and optically dark galaxies[END_REF] and 16 galaxies within 3.5 and 4.8 ‡ detected using ancillary information, mainly Spitzer -IRAC prior positions [START_REF] Franco | GOODS-ALMA: Using IRAC and VLA to probe fainter millimeter galaxies[END_REF]. The median redshift and stellar mass of the S/N Ø 4.8 sources are z = 2.73 and M ı = 1.0 ◊ 10 11 M § , whereas the sources with a 4.8 > S/N Ø 3.5, are both slightly closer (z = 2.40) and less massive (M ı = 7.2 ◊ 10 10 M § ).

Compared with other blind surveys of dust continuum [START_REF] Walter | ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: Survey Description[END_REF][START_REF] Dunlop | A deep ALMA image of the Hubble Ultra Deep Field[END_REF][START_REF] Hatsukade | ALMA twenty-six arcmin 2 survey of GOODS-S at one millimeter (ASAGAO): Source catalog and number counts[END_REF][START_REF] Decarli | The ALMA Spectroscopic Survey in the HUDF: CO Luminosity Functions and the Molecular Gas Content of Galaxies through Cosmic History[END_REF], GOODS-ALMA is the most efficient to probe distant galaxies, not only because it covers the largest area, but also because of the strategy to observe a shallower but wider field within a limited amount of time. This is also confirmed by the flattening at ≥ 0.1 mJy in the observed 1.1 mm number counts [START_REF] Muñoz Arancibia | The ALMA Frontier Fields Survey. IV. Lensing-corrected 1.1 mm number counts in Abell 2744[END_REF][START_REF] González-López | The ALMA Spectroscopic Survey in the HUDF: Deep 1.2 mm Continuum Number Counts[END_REF][START_REF] Popping | The ALMA Spectroscopic Survey in the HUDF: A Model to Explain Observed 1.1 and 0.85 mm Dust Continuum Number Counts[END_REF]. The significant decrease of galaxies fainter than 0.1 mJy suggests that deep pencil-beam surveys would be less efficient in searching for the sources contributing to the cosmic infrared background. Despite being counter-intuitive, this can be understood by the fact that submillimeter detected galaxies tend to be the most massive ones, which are rare and bright. 2017) survey covering all of the Hubble Ultra Deep Field region, the yellow contours show the ASAGAO survey [START_REF] Hatsukade | ALMA twenty-six arcmin 2 survey of GOODS-S at one millimeter (ASAGAO): Source catalog and number counts[END_REF], the gray contours show the ASPECS Pilot survey [START_REF] Walter | ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: Survey Description[END_REF], the pink contours show the ASPECS Large Program [START_REF] Decarli | The ALMA Spectroscopic Survey in the HUDF: CO Luminosity Functions and the Molecular Gas Content of Galaxies through Cosmic History[END_REF].

Optically-dark galaxies

As mentioned in Section 1.2.3, in pre-ALMA era, our understanding of the cosmic star formation history at z > 3 was limited to the rest-frame UV observed galaxies which are now known to under-represent the most massive and star-forming galaxies. For example, it was over a decade after the discovery of the brightest submillimeter source in the Hubble Deep Field (Hughes et al., 1998, HDF850.1, detected using SCUBA) that [START_REF] Walter | The intense starburst HDF 850.1 in a galaxy overdensity at z ¥ 5.2 in the Hubble Deep Field[END_REF] finally identified with the molecular emission lines from ALMA observations that it is a starburst galaxy at z = 5.183, and invisible in shorter wavelength. In order to better constrain the cosmic star formation in the first two billion years (z > 3) with a statistically large 1.2 Star formation obscured by interstellar dust sample of optically-dark galaxies, [START_REF] Wang | A dominant population of optically invisible massive galaxies in the early Universe[END_REF] pre-selected 63 optically-dark galaxies based on their NIR-MIR colors (see also in Section 1.2.1) and followed them up with ALMA. They found that these galaxies contribute ten times more to the cosmic SFRD at z > 3 compared with equivalently massive UV-bright galaxies (Fig. 1.10-Left). Different from the classic SMGs, these galaxies show an average SFR of 200 M § yr ≠1 , typical of main sequence galaxies at these redshifts although with a large scatter. On the other hand, [START_REF] Wang | A dominant population of optically invisible massive galaxies in the early Universe[END_REF] derived the clustering properties of these optically-dark galaxies from their cross-correlation with optically-detected galaxies with similar redshifts and masses. They found that these galaxies constitute excellent candidate progenitors of the most massive galaxies presently located in the center of massive groups and galaxy clusters. Comparing the contribution of these galaxies to the cosmic SFRD (Fig. 1.10-Left) with the one of the galaxies from protocluster cores predicted by simulations [START_REF] Chiang | Galaxy Protoclusters as Drivers of Cosmic Star Formation History in the First 2 Gyr[END_REF] (Fig. 1.10-Right), both populations show a similar contribution of ≥ 10 ≠3 at 3 < z < 6, enhancing their association with large scale structures. This has been also indicated by the multiplicity of submillimeter galaxies. The observed high number density of submillimeter galaxies relative to that derived from blank-field counts suggests that at least 30% of the multiplicity are physically associated galaxies rather than due to pure projection effect [START_REF] Simpson | The SCUBA-2 Cosmology Legacy Survey: ALMA Resolves the Bright-end of the Sub-millimeter Number Counts[END_REF][START_REF] Stach | An ALMA Survey of the SCUBA-2 Cosmology Legacy Survey UKIDSS/UDS Field: Number Counts of Submillimeter Galaxies[END_REF].

As the largest blind survey with ALMA, the GOODS-ALMA survey also overlaps the region with the deepest HST-WFC3 H-band imaging, down to a 5 ‡ depth of H = 28.2 AB [START_REF] Guo | CANDELS Multi-wavelength Catalogs: Source Detection and Photometry in the GOODS-South Field[END_REF]. Out of the 35 GOODS-ALMA detections, an ensemble of six galaxies are optically-dark since they are not detected in the optical to near-infrared HST images including the deep H-band image. In Chapter 3, I will show my work on the detailed study of these galaxies, especially a large-scale structure they are tracing.

14

Chapter 1 Introduction

Star formation fueled by gas

As the key ingredient of star formation, the gas component is vital to our understanding of galaxy evolution. The physical processes involving gas span a vast range of physical scales: from the accretion of intergalactic gas (≥ Mpc), to the settling down and the cooling in galaxies (≥ kpc), to the collapse into molecular clouds (≥ 10-100 pc), then to the fragmentation into denser cores (≥ 0.1-1 pc), finally to the formation of stars (see review by [START_REF] Kennicutt | Star Formation in the Milky Way and Nearby Galaxies[END_REF].

In extragalactic astrophysics, we are limited by observations to the processes down to sub-kpc scale, which leads to mostly statistical studies on the gas fraction, star formation efficiency/gas depletion time, etc.

Gas tracers

Atomic gas, HI, is traced by the hyperfine transition of hydrogen, at 21 cm (rest-frame). Molecular gas, H 2 , is more complex because the H 2 molecule is symmetric and has no dipolar transition. The quadrupolar transitions can only be excited in gas with T & 100 K, which is not possible for gas in the cold molecular clouds where star formation takes place. The second most abundant molecule, CO, can be easily excited in cold molecular clouds. It is then used to trace the bulk distribution of molecular gas using a conversion factor from CO luminosity to H 2 mass, M mol = -CO L Õ CO , or from CO line intensity to H 2 column density, N H2 = X CO W (CO) [START_REF] Bolatto | The CO-to-H2 Conversion Factor[END_REF]. On the other hand, dust is also a good tracer of molecular gas (e.g., [START_REF] Magdis | The Evolving Interstellar Medium of Star-forming Galaxies since z = 2 as Probed by Their Infrared Spectral Energy Distributions[END_REF][START_REF] Magnelli | The deepest Herschel-PACS far-infrared survey: number counts and infrared luminosity functions from combined PEP/GOODS-H observations[END_REF][START_REF] Genzel | Combined CO and Dust Scaling Relations of Depletion Time and Molecular Gas Fractions with Cosmic Time, Specific Star-formation Rate, and Stellar Mass[END_REF]. The gas mass (H 2 and HI) can be derived by fitting the far-IR to submillimeter SEDs (e.g. Draine et al., 2007) with dust models and adopting a dust-to-gas mass ratio (e.g. [START_REF] Leroy | The CO-to-H2 Conversion Factor from Infrared Dust Emission across the Local Group[END_REF]. [START_REF] Scoville | The Evolution of Interstellar Medium Mass Probed by Dust Emission: ALMA Observations at z = 0.3-2[END_REF], [START_REF] Scoville | ISM Masses and the Star formation Law at Z = 1 to 6: ALMA Observations of Dust Continuum in 145 Galaxies in the COSMOS Survey Field[END_REF][START_REF] Scoville | Evolution of Interstellar Medium, Star Formation, and Accretion at High Redshift[END_REF] recently proposed to convert a single photometric measurement in the Rayleigh-Jeans tail (⁄ > 100 µm) to gas mass with a conversion factor -850µm assuming the dominant dust temperature is 25 K and dust emission is optically thin. Moreover, molecules like HCN, HCO + , CS are tracers of denser molecular gas in the cores of star-forming clouds.

Molecular gas is extremely challenging to detect in metal poor galaxies even if some of them do show ongoing star formation. Their low metallicity indicates low dust content, which allows far-UV photons to penetrate deeper in the molecular clouds, as illustrated in Figure 1.11. Compared to H 2 molecules which can effectively self-shield from the photodissociating far-UV photons, CO molecules are more fragile and can be easily dissociated into C atoms and C + ions. Therefore, there would be a large fraction of the so-called "CO-dark" molecular gas outside the CO-emitting core. [CII] is much brighter than CO in such metal poor galaxies [START_REF] Cormier | The Herschel Dwarf Galaxy Survey. I. Properties of the low-metallicity ISM from PACS spectroscopy[END_REF], the "CO-dark" H 2 gas could then be traced by [CII] [START_REF] Madden | Tracing the total molecular gas in galaxies: [CII] and the CO-dark gas[END_REF]. However, knowing that [CII] emission originates not only from neutral photodissociation regions (PDRs), but also from ionised gas, the calibration from [CII] to H 2 gas mass requires a precise estimation of the contribution from both origins, which is still unclear [START_REF] Cormier | The Herschel Dwarf Galaxy Survey. II. Physical conditions, origin of [C II] emission, and porosity of the multiphase low-metallicity ISM[END_REF]Accurso et al., 2017a). I will present my work on the search for molecular gas in one of the most metal poor galaxies, IZw18, in Chapter 4.

Star formation fueled by gas

Star formation law

The well-known scaling relation between gas column density and star formation rate surface density, SFR Ã N gas , was established based on the observation of the Milky Way and nearby galaxies (see e.g., Figure 1.12, Kennicutt, 1998b;[START_REF] Bigiel | The Star Formation Law in Nearby Galaxies on Sub-Kpc Scales[END_REF] and it stands also for more distant galaxies [START_REF] Daddi | Different Star Formation Laws for Disks Versus Starbursts at Low and High Redshifts[END_REF][START_REF] Tacconi | Phibss: Molecular Gas Content and Scaling Relations in z ~1-3 Massive, Main-sequence Star-forming Galaxies[END_REF]. This relation shows how efficiently galaxies turn their gas into stars and constrains theoretical models.

The disk averaged star formation law has been studied extensively, from local galaxies to high-z galaxies, from normal star-forming galaxies to starbursts (see reviews by Kennicutt, 1998b;[START_REF] Kennicutt | Star Formation in the Milky Way and Nearby Galaxies[END_REF]. A non-linear slope (N in SFR à N gas , where gas is the total gas (H 2 and HI) column density) of ≥ 1.4 is found in the tight scaling relation, with SFR spanning almost five orders of magnitude (Figure 1.12). Different normalizations of the scaling relation are also found in high-z starbursts [START_REF] Daddi | Different Star Formation Laws for Disks Versus Starbursts at Low and High Redshifts[END_REF] and metal-poor galaxies, but these results suffer from the infamous uncertainty of the CO-to-H 2 conversion factor which is related to the metallicity in galaxies and this relation is still unclear [START_REF] Bolatto | The CO-to-H2 Conversion Factor[END_REF]. However, [START_REF] Tacconi | The Evolution of the Star-forming Interstellar Medium across Cosmic Time[END_REF] found that the normalization is indeed related to the starburstiness (" MS © SFR/SFR MS ) of a galaxy, as à (" MS) 0.44 , based on their study on the large sample of 14444 star-forming galaxies at 0 < z < 4. The low gas masses in the galaxies above the main sequence are explained by their dependence on (" MS) 0.44 , instead the difference in -CO . [START_REF] Bigiel | The Star Formation Law in Nearby Galaxies on Sub-Kpc Scales[END_REF] spatially resolved seven nearby spiral galaxies down to 750 pc 11 and studied the atomic, molecular and total gas -star formation law in Figure 1.12. While there is little correlation between atomic gas and star formation, HI shows a clear saturation effect at ≥ 9 M § pc ≠2 (Figure 1.12a, vertical dashed line). The gas with high surface density tends to be in the molecular phase. This saturation is not seen in molecular gas, and H2 shows a good linear relation with SFR i.e., efficiency (SFE © SFR/M H2 ), and equivalently, a constant gas depletion time (• dep © M H2 /SFR) of ≥2 Gyr. This is interpreted as evidence that star formation proceeds in a uniform way in giant molecular clouds in spiral galaxies [START_REF] Bigiel | The Star Formation Law in Nearby Galaxies on Sub-Kpc Scales[END_REF][START_REF] Bigiel | A Constant Molecular Gas Depletion Time in Nearby Disk Galaxies[END_REF]. They also found that the SFE declines with radius and dwarf galaxies have similar SFEs to the outskirts of spirals as they both have low gas density and are HI dominated. Combining with recent results of the high redshift galaxies, it is found that SFE increases very slightly with redshift and is only weakly dependent on stellar mass, as SFE ≥ (1 + z) 0.6≥1 , about a factor of 2 from z = 0 to z = 2.5 [START_REF] Tacconi | Phibss: Molecular Gas Content and Scaling Relations in z ~1-3 Massive, Main-sequence Star-forming Galaxies[END_REF][START_REF] Tacconi | PHIBSS: Unified Scaling Relations of Gas Depletion Time and Molecular Gas Fractions[END_REF]. Compared with the long timescale to accumulate stellar mass in galaxies (M star /SFR) indicated by the main sequence as mentioned in Section 1.2.2, the short molecular gas depletion timescale implies that external gas replenishment is needed to sustain star formation in galaxies. The cold stream framework proposed by simulations predict cold gas accretion rates proportional to the host halo mass and evolving with redshift (e.g, [START_REF] Dekel | Cold streams in early massive hot haloes as the main mode of galaxy formation[END_REF], that leads to an agreement with the observed galaxy main sequence behaviour. However, the cold streams, i.e., the cold gas inflows, has been extremely hard to observe. Tentative detections through extended Ly-emission in galaxy clusters have been obtained by [START_REF] Daddi | Three Lyman-alpha emitting filaments converging to a massive galaxy group at z=2.91: a case for cold gas infall[END_REF] and [START_REF] Ao | Infalling gas in a Lyman--blob[END_REF]. Although it is hard to confirm whether such emission originates from collisionally excited gas infall, the study of such structures can still put valuable constraints on the presence of gas inflows.

Gas content

As mentioned in Chapter 1.2.2, for galaxies at a specific mass, their SFRs increase with redshift (Figure 1.4), i.e., the galaxy main sequence evolves with redshift, as sSFR Ã (1 + z) 3 until z ≥ 2, and sSFR Ã (1 + z) 1.5 at z > 2 [START_REF] Lilly | Gas Regulation of Galaxies: The Evolution of the Cosmic Specific Star Formation Rate, the Metallicity-Mass-Star-formation Rate Relation, and the Stellar Content of Halos[END_REF], but saturates at z ≥ 4 [START_REF] Schreiber | The Herschel view of the dominant mode of galaxy growth from z = 4 to the present day[END_REF]. As the fuel of star formation, the cosmic evolution of the gas content, which is related to both the specific SFR and the gas depletion time, f gas © Mgas Mstar = sSFR ◊ • dep , is also important for us to test theory and models of star formation in galaxies. The dependence of sSFR and • dep on redshift lead to a steep increase of gas fraction until z ≥ 2 -3, from f gas ≥ 10% in the local Universe to the highest value of f gas ≥ 60% at z ≥ 3 (Figure 1.13). We note that these are the averaged behaviour of galaxies, and individual galaxies can have higher or lower gas fractions. [START_REF] Tacconi | The Evolution of the Star-forming Interstellar Medium across Cosmic Time[END_REF] found that gas content not only evolves with redshift, but also is related to the starburstiness and stellar mass, ≠0.36 . They suggest that the drop of gas content seen in the high stellar mass end, as is also seen in sSFR, i.e., the fattening at the high mass end in galaxy main sequence, indicates a "mass" quenching process in the most massive galaxies. 1.4 Star formation as one of the energy sources of gas 1.4.1 Gas velocity dispersion and the energy sources Galaxies at different cosmic epochs show quite distinct properties. Compared to their high-redshift counterparts at similar stellar masses, local star-forming galaxies are larger, and have relatively lower gas fractions and lower SFRs [START_REF] Leroy | The Molecular Interstellar Medium of Dwarf Galaxies on Kiloparsec Scales: A New Survey for CO in Northern, IRAS-detected Dwarf Galaxies[END_REF]Daddi et al., 2010a;[START_REF] Tacconi | High molecular gas fractions in normal massive star-forming galaxies in the young Universe[END_REF][START_REF] Madau | Cosmic Star-Formation History[END_REF]. They are also less likely to experience violent events such as major mergers and gas accretion [START_REF] Baugh | Evolution of the Hubble sequence in hierarchical models for galaxy formation[END_REF][START_REF] Genzel | From Rings to Bulges: Evidence for Rapid Secular Galaxy Evolution at z ˜2 from Integral Field Spectroscopy in the SINS Survey[END_REF][START_REF] Robotham | Galaxy And Mass Assembly (GAMA): galaxy close pairs, mergers and the future fate of stellar mass[END_REF]. Despite of all the various properties, galaxy discs at all epochs tend to be in a state of marginal gravitational stability, which can be characterized by the close to unity [START_REF] Toomre | Discovery of Protoclusters at z ≥ 3.7 and 4.9: Embedded in Primordial Superclusters[END_REF] Q parameter, as Q = Ÿ ‡/fiG , where Ÿ is the epicyclic frequency of the galaxy's rotation, ‡ the velocity dispersion, represents the effect of pressure, and the mass surface density, represents the effect of gravity. However, many theoretical and observational studies suggest that gas in higher-z galaxies has larger random motions compared to gas in nearby galaxies. Galaxies at z > 1 have velocity dispersions in the range of 50-100 km s ≠1 [START_REF] Nesvadba | Extreme Gas Kinematics in the z=2.2 Powerful Radio Galaxy MRC 1138-262: Evidence for Efficient Active Galactic Nucleus Feedback in the Early Universe?[END_REF][START_REF] Lehnert | Physical Conditions in the Interstellar Medium of Intensely Star-Forming Galaxies at Redshift˜2[END_REF][START_REF] Lehnert | On the self-regulation of intense star-formation in galaxies at z = 1-3[END_REF][START_REF] Schreiber | The SINS Survey: SINFONI Integral Field Spectroscopy of z ˜2 Star-forming Galaxies[END_REF][START_REF] Wisnioski | The KMOS 3D Survey: Design, First Results, and the Evolution of Galaxy Kinematics from 0.7 = z = 2.7[END_REF] and show an almost linear correlation with the SFR, while local galaxies show typical velocity dispersions of < 50 km s ≠1 [START_REF] Varidel | Resolved Gas Kinematics in a Sample of Low-Redshift High Star-Formation Rate Galaxies[END_REF][START_REF] Yu | What drives the velocity dispersion of ionized gas in star-forming galaxies?[END_REF]. On the other hand, both local and high-z galaxies show velocity dispersions higher than expected from only the thermal contribution of gas. The characteristic temperature of 10 4 K corresponds to a typical velocity dispersion of ≥9 km s ≠1 for the ionized gas emitting at H- [START_REF] Glazebrook | The Dawes Review 1: Kinematic Studies of Star-Forming Galaxies Across Cosmic Time[END_REF].

f gas à (1 + z) 2.5 ◊ (" MS) 0.52 ◊ (M star )
The dominant energy source of the non-thermal turbulent motions is unclear. Numerous drivers have been proposed, including star formation feedback (Mac [START_REF] Low | Control of star formation by supersonic turbulence[END_REF][START_REF] Krumholz | The Dynamics of Radiation-pressure-dominated H II Regions[END_REF][START_REF] Murray | The Disruption of Giant Molecular Clouds by Radiation Pressure & the Efficiency of Star Formation in Galaxies[END_REF], radial transport of gas in discs due to gravitation (Krumholz and Burkhart, 2016;[START_REF] Krumholz | A unified model for galactic discs: star formation, turbulence driving, and mass transport[END_REF], gas accretion from the intergalactic medium and minor mergers [START_REF] Dekel | Cold streams in early massive hot haloes as the main mode of galaxy formation[END_REF][START_REF] Glazebrook | The Dawes Review 1: Kinematic Studies of Star-Forming Galaxies Across Cosmic Time[END_REF], galactic shear from the differential rotation in disc galaxies [START_REF] Krumholz | Is Turbulence in the Interstellar Medium Driven by Feedback or Gravity? An Observational Test[END_REF][START_REF] Federrath | The Link between Turbulence, Magnetic Fields, Filaments, and Star Formation in the Central Molecular Zone Cloud G0.253+0.016[END_REF][START_REF] Zhou | The SAMI Galaxy Survey: energy sources of the turbulent velocity dispersion in spatially resolved local star-forming galaxies[END_REF], etc. In Chapter 5, I studied the driver of turbulent motion of eight spatially-resolved nearby star-forming galaxies. The results show that star formation feedback is not the main energy source of the turbulent motions in galaxies with low SFR surface density. However, recent studies by [START_REF] Krumholz | A unified model for galactic discs: star formation, turbulence driving, and mass transport[END_REF] and [START_REF] Varidel | The SAMI galaxy survey: gas velocity dispersions in low-z star-forming galaxies and the drivers of turbulence[END_REF] on the global properties of galaxies found that the models taking into consideration of both the star formation feedback and the gravitational energy release from radial transport of gas can yield excellent agreement with the observations of galaxies with SFR ranging from 10 ≠4 M § yr ≠1 to 10 3 M § yr ≠1 . The model predicts a transition from gravity-dominantly-driven turbulence in high-z galaxies to starformation-driven turbulence in low-z galaxies, where SFR is lower. The distinct conclusions come from the different treatments of beam smearing effects, as will be explained below. This model also explains why galaxy bulges tend to form at high redshift and discs at lower redshift, and why galaxies tend to quench inside-out, because the gas accretion rate increases much faster with velocity dispersion, than SFR with velocity dispersion ( Ṁacc à ‡ 3 gas vs. SFR à ‡ gas ) and then masses are transported more inward to a bulge in high-z galaxies and remain in the outskirts to form a disk in low-z galaxies [START_REF] Krumholz | A unified model for galactic discs: star formation, turbulence driving, and mass transport[END_REF].

Integral field unit (IFU), datacube and beam smearing e ect

The integral field unit technique has been widely used in optical and near-IR to study galaxy kinematics. It allows us to obtain spatially resolved spectral information in the galaxies. In Chapter 5, I will present my work on the energy sources of the turbulent motions in local star-forming galaxies making use of the IFU survey, the SAMI Galaxy Survey [START_REF] Croom | The Sydney-AAO Multi-object Integral field spectrograph[END_REF][START_REF] Bryant | The SAMI Galaxy Survey: instrument specification and target selection[END_REF]. The three dimensional information from such IFU observations are saved in the so-called datacubes (Figure 1.14). This is similar to the data product obtained from the interferometers at submillimeter to radio wavelengths.

Measurements of velocity dispersions from these datacubes are limited by the spectral and spatial resolution of the instruments. The observed emission line is broadened by the spectral resolution, but this can be addressed by convolving the line spread function into the emission line fitting. The limited spatial resolution blurs the spatial distribution of the intrinsic flux, the line of sight velocity profile, and 1.4 Star formation as one of the energy sources of gas the line of sight velocity dispersion within the smallest resolved area. As a result, the observed velocity dispersion is elevated with the unresolved velocity gradient. This is the so-called beam smearing effect. Several tools have been developed to account for beam-smearing effect, e.g., 3D BBAROLO 12 (DiTeodoro and Fraternali, 2015),GBKFIT13 [START_REF] Bekiaris | Kinematic modelling of disc galaxies using graphics processing units[END_REF], GALPAK 3D14 [START_REF] Bouché | GalPak 3D : A Bayesian Parametric Tool for Extracting Morphokinematics of Galaxies from 3D Data[END_REF], and BLOBBY3D [START_REF] Varidel | The SAMI Galaxy Survey: Bayesian inference for gas disc kinematics using a hierarchical Gaussian mixture model[END_REF]. They construct a 3D modelled cube for the galaxy and then spatially convolve the cube per spectral slice to simulate the effect of beam smearing. The convolved cube is finally compared to the observed data.

During the five years of my PhD, I have worked on several different projects described below. This thesis was done within a collaboration between Nanjing University, China and CEA-Saclay, France. I am also involved in several international research teams including GOODS-ALMA (PI: D. Elbaz), VANDELS (PIs: L. Pentericci and R. McLure) and SAMI (Team leader: S. Croom). I have published as first-author three papers and I have received the referee report of the forth paper while writing this manuscript. As principle or collaborating investigator, I have four observation projects accepted by ALMA, NOEMA and CFHT. I also presented my work in several international conferences with four talks and four posters. A full list of my publications, proposals and talks can be found in Appendix ??.

A summary of my work done during this thesis is given as follows:

2.1 GOODS-ALMA I have been working on the GOODS-ALMA survey (Project ID: 2015.1.00543.S, 2017.1.00543.S) during the last three years. This project aims at opening a new window on the dusty star formation of galaxies in the early Universe, by carrying out a wide (69 arcmin 2 ) and deep enough (180µJy at 0 ÕÕ .6) survey at 1.1 mm to detect dusty galaxies efficiently, as introduced in Section 1.2.4. Based on our analysis on the ALMA Cycle 3 data (2015.1.00543.S), the detections in the GOODS-ALMA survey show 1) a population of optically dark galaxies that make up 17% of the detections and 2) a high fraction, 40%, of AGNs [START_REF] Franco | GOODS-ALMA: 1.1 mm galaxy survey. I. Source catalog and optically dark galaxies[END_REF].

Owing to the abundant ancillary data in the GOODS-South field, I made use of multi-wavelength observations to derive the properties of the optically dark galaxies and compiled the existing catalogs (ZFOURGE, CANDELS, CDFS-7Ms) to study the submillimeter emission of different galaxy populations by stacking.

2.1.1 Optically dark galaxies and their association with a proto-cluster in formation at z ≥ 3.5 I studied thoroughly the properties of the six optically dark galaxies detected in the GOODS-ALMA 1.1 mm continuum survey. None of them is listed in the deepest H-band based CANDELS catalog in the GOODS-South field. Five of them suffer from the confusion with bright neighboring galaxies even in the highly resolved optical to near-IR images. To retrieve information at these wavelengths, I performed a de-blending procedure with the method developed by one of the co-authors, C. Schreiber (Schreiber et al., 2018a, code). It turns out that after this deblending analysis two out of the six galaxies end up having H-band counterparts with magnitudes brighter than the detection limit determined by in the CANDELS team on their catalog. They were missed because they were considered as part of the neighboring galaxy because of the confusion limit as a drawback of the source extraction procedure. I fitted the optical-to-MIR SEDs of the optically dark galaxies and their neighbors respectively. The derived redshifts confirm the confusion due to a projection effect, meaning that the optically dark galaxies and their neighbors are at different redshifts.

As co-I, I contributed to a spectroscopic follow-up using ALMA (2018.1.01079.S, PI: M. Franco) to identify the exact redshifts of these optically dark galaxies. I analyzed the data with CASA and GILDAS and found that one emission line was detected in two of the galaxies.

To study the environment where these galaxies reside in, I constructed the surface number density map of galaxies in the GOODS-South field based on the ZFOURGE catalog. We choose ZFOURGE rather than CANDELS because the ZFOURGE catalog is based on detections extracted from the near-IR K s band images, where galaxies at higher redshift appear brighter than they are in the optical images. We present evidence that nearly 70 % of the optically dark galaxies belong to the same over-density of galaxies at z ≥ 3.5. We also found that the most massive one of the optically dark galaxies is also the most massive galaxy at z > 3 in the GOODS-ALMA field after excluding galaxies hosting a luminous AGN potentially responsible for an overestimation of their stellar mass. This galaxy, AGS24, falls at the very center of the peak of the galaxy surface density. This suggests that the surrounding over-density is a proto-cluster in the process of virialization and this massive galaxy is the candidate progenitor of the future Brightest Cluster Galaxy (BCG).

These optically dark galaxies unveiled by ALMA are good tracers of such large-scale structures in the early Universe and they can serve to test current theories on the formation of the most massive galaxies during the first billin years of the Universe.

Data compilation

The ALMA Cycle 5 data (2017.1.00543.S) fully arrived at the end of my PhD. The Cycle 3 and the Cycle 5 data share almost the same observation settings but the angular resolutions, meaning that they both reach a sensitivity of ≥ 128 µJy at their natural resolutions of ≥ 0 ÕÕ .2 and ≥ 1 ÕÕ .4, respectively. Our preliminary analysis has shown that in the low resolution image (Cycle 5), there are apparently more detections than in the high resolution image (Cycle 3). However, most of the new detections are galaxies at lower redshift z . 2. Source extraction on the new data is led by another team member, C. Gomez Guijarro. I took part in merging the two sets of GOODS-ALMA data to push down the sensitivity to ≥ 85 µJy at the combined natural resolution of ≥ 0 ÕÕ .45. The preliminary flux measurements in the low resolution data and the merged data also show differences compared with those obtained in the high resolution data from Cycle 3. We tried to taper this merged image to different resolutions to see the variance of flux of the detections. Further analysis is led by M. Xiao in our team. In addition to the two sets of GOODS-ALMA data, there are also ALMA archive data overlapping this field including deep surveys like ASPECS (PI: F. Walter), ASAGAO (PI: K. Kohno), HUDF (PI: J. Dunlop). I have also contributed to develop the procedure to create a "wedding cake" map combining all these observations. ALMA being an interferometer, it provides us with 3D-datacubes, where spectroscopic information is also available at all the sky positions of the mosaic image. I have collaborated with the ASPECS team to search for emission lines in the ALMA data when I started to work on GOODS-ALMA. However, the source extraction from spectra always requires much higher sensitivity than that from continuum and indeed the Cycle 3 data alone is far from enough to retrieve meaningful information. With the "wedding cake" 3D map, we can start from the brightest detections in the dust continuum and try again to search for line emission. This is also one of my future plans.

Nascent AGNs in GOODS-ALMA

Despite the high fraction of AGNs, 40%, found in the detections in the GOODS-ALMA field, we notice that half of the AGNs show very high infrared to radio flux ratios, which goes against the existence of AGNs. The contradiction leads to two different paths -a) These AGNs at high redshift are so heavily attenuated that the re-emission by dust makes them more visible by ALMA. b) A larger amount of X-ray binaries are produced during star formation in the galaxies at high redshift and emit significantly in the X-ray domain, so the galaxies are confounded as AGNs.

Studies on the X-ray and infrared properties of star-forming galaxies are limited to galaxies at z . 2 [START_REF] Mineo | X-ray emission from star-forming galaxies -II. Hot interstellarmedium[END_REF][START_REF] Symeonidis | Linking the X-ray and infrared properties of star-forming galaxies at z < 1.5[END_REF][START_REF] Rodighiero | Active Galactic Nuclei in Dusty Starbursts at z = 2: Feedback Still to Kick in[END_REF]. The high angular resolution and sensitivity of the GOODS-ALMA data enable us to perform stacking to study more distant galaxies. Taking into account both the ZFOURGE catalog, which is more comprehensive on the high-redshift galaxies, and the CDFS-7Ms catalog, I compared the star-forming galaxies with and without an AGN at 2 < z < 4. My first attempt with the Cycle 3 data alone did not yield meaningful results. However, with the merged image (Cycle 3 and Cycle 5), I found that AGNs have up to 50% higher flux densities at 1.1 mm than purely star-forming galaxies with similar stellar masses. This indicates that AGNs do not decrease the SFRs hence do not quench these star-forming galaxies, since even at these redshifts the 1.1 mm flux is largely dominated by dust heated by young and massive stars. We note that these are not QSO's, and their FIR SEDs are typical of those of star-forming galaxies. This is comparable to the excess of AGNs among starbursts at z = 2 in [START_REF] Rodighiero | Active Galactic Nuclei in Dusty Starbursts at z = 2: Feedback Still to Kick in[END_REF]. I will keep working on that to see the correlation between X-ray and infrared emission in these galaxies.

Contribution as third author

I have contributed to several papers as co-author. In particular, I have contributed as third author in [START_REF] Franco | GOODS-ALMA: The slow downfall of star-formation in z = 2-3 massive galaxies[END_REF] and [START_REF] Franco | GOODS-ALMA: Using IRAC and VLA to probe fainter millimeter galaxies[END_REF] on GOODS-ALMA. In [START_REF] Franco | GOODS-ALMA: The slow downfall of star-formation in z = 2-3 massive galaxies[END_REF], we created a supplementary catalog using IRAC (mid-IR) and VLA (radio) priors to identify detections from 4.8 ‡ down to 3.5 ‡. In [START_REF] Franco | GOODS-ALMA: Using IRAC and VLA to probe fainter millimeter galaxies[END_REF], we studied the properties of the detections. One of the main results is that the submillimeter sizes of the GOODS-ALMA detections are smaller than their H-band sizes which are compatible with the typical z ≥ 2 elliptical galaxies at similar masses. This indicates that compact bulges are forming in these galaxies.

Extremely metal-poor galaxies

The low dust content of the local extremely metal poor galaxies is similar to the normal star-forming galaxies found at high-z (z>5, e.g. [START_REF] Walter | Evidence for Low Extinction in Actively Star-forming Galaxies at z > 6.5[END_REF][START_REF] Capak | Galaxies at redshifts 5 to 6 with systematically low dust content and high [C II] emission[END_REF], which makes them ideal local analogs to study the first galaxies in the early Universe.

Spatially resolved dust emission

This work has been published in the Monthly Notices of the Royal Astronomical Society [START_REF] Zhou | Spatially resolved dust emission of extremely metal-poor galaxies*[END_REF], and is presented in Chapter 6.

I started my PhD with the study of the spatially resolved dust emission of extremely metal-poor galaxies. Our spatially resolved images from Herschel reveal higher dust temperatures and lower emissivity indices (-; using the single modified blackbody (MMB) model) compared to that of Spirals. And the two MMB fits with a fixed -=2 show that at 100 µm, about half of the emission comes from warm (50 K) dust, in contrast to the cold (≥20 K) dust component. The far-IR colours are all related to the surface densities of young stars as traced by far-UV, 24 µm and SFRs, but not to the stellar mass surface densities. This suggests that the dust emitting at wavelengths from 70 to 350 µm is primarily heated by radiation from young stars.

Gas content in IZw18

I submitted a letter to Astronomy & Astrophysics to report this work. The referee report has been recently returned. This work is presented in Chapter 4.

The predicted CO flux in high-z normal star forming galaxies is in the order of µJy, challenging even with ALMA. Therefore, detailed studies on the metal-poor interstellar medium (ISM) rely on their local analogs. As co-I, I observed the CO J=2-1 emission in one of the most metal poor galaxies, IZw18, with NOEMA (W17BT, W18CA). I performed the data reduction with GILDAS and CASA. Based on the observation from the upgraded NOEMA Interferometer, we obtain an upper limit on the CO J=2-1 emission for IZw18 of L Õ CO(2≠1) < 7.64 ◊ 10 3 K km s ≠1 pc ≠2 , ten times lower than previous studies (e.g., [START_REF] Leroy | The Low CO Content of the Extremely Metal-poor Galaxy I Zw 18[END_REF]. As one of the most metal poor galaxies, IZw18 shows extremely low CO content despite its vigorous star formation activity. Such low CO content relative to its infrared luminosity, star formation rate and [C II] luminosity, compared to other galaxies, indicates a drastic change in the structure of the ISM at around a few percentages of Solar metallicity. Especially, the high [C II] luminosity relative to CO implies a larger molecular reservoir than the CO emitter in IZw18.

The SAMI Integral field Units (IFU) Survey

Similar to ALMA, IFU observations also produce 3D datacubes. Then we can associate the physical properties extracted from spectra, with their spatial information to understand the underlying physical processes. I collaborated with F. Bian, T. Yuan, C. Federrath, A. Melding and other members in the SAMI survey team to study spatially resolved nearby star-forming galaxies.

Energy source of turbulence in star-forming galaxies

This work has been published in Monthly Notices of the Royal Astronomical Society [START_REF] Zhou | The SAMI Galaxy Survey: energy sources of the turbulent velocity dispersion in spatially resolved local star-forming galaxies[END_REF] and is presented in Chapter 5.

I studied the energy sources of the turbulent velocity dispersion in spatially resolved local star-forming galaxies based on the SAMI survey. We found that on sub-kpc scales, our galaxies display a flat distribution of ionized gas velocity dispersions as a function of SFR surface density. A major fraction of our galaxies shows higher velocity dispersions than the predictions by feedback-driven models, especially at the low SFR surface density end. Our results suggest that additional sources beyond star formation feedback contribute to driving random motions of the interstellar medium in star-forming galaxies.

Observations

At the same time, I proposed as principle and collaborating investigator several observation projects to state-of-the-art telescopes. The results obtained from some of these proposals (ALMA: 2018.1.01079.S; NOEMA: W17BT, W18CA) have already been presented in some of the publications as mentioned above (Zhou et al., 2020, Zhou et al. submitted). In the following, I briefly describe the remaining proposals that I am involved in.

ALMA: Upon the discovery of the optically dark galaxies, we have been trying to propose ALMA observations to obtain spectroscopic redshifts for these galaxies. We have obtained one emission line in two galaxies from our first spectroscopic follow-up of the optically dark galaxies (2018.1.01079.S). Now that we have made a solid case that four (maybe even five) of them fall in the same large-scale structure, we will propose to observe the brightest line, [C II] of these galaxies to derive precise redshifts and study the properties associated with this line, e.g, the SFR-[C II] relation, the possible molecular gas content indicated by this line and the spatial distribution of [C II] emission since the observation would cover the entire overdensity.

VLA: I have attempted to obtain redshifts as well from the CO(1-0) emission in the optically dark galaxies with VLA. However, VLA was not ideal for redshift measurements as it does not cover as wide a frequency range as ALMA. The proposal was finally rejected. Once we have the accurate spectroscopic redshifts, we will come back to request CO(1-0) observations, as this transition traces better the molecular gas content than the higher-J transitions available in the ALMA windows. In addition, [START_REF] Ginolfi | Molecular gas on large circumgalactic scales at z = 3.47[END_REF] already showed evidence of a CO gas stream surrounding one of the optically-detected galaxies in this structure. We expect the existence of molecular gas in not only the galaxies, but also across the entire structure.

CFHT:

In order to understand the low star formation efficiency in extremely metal poor galaxies, we need information on the molecular gas content in such galaxies. I proposed to observe the hot H 2 emission in Sextans A with WIRCam of CFHT. If detected, we may also study its spatial distribution and the interaction with the star formation activity within the galaxy. This proposal was approved on May 5, 2020 and graded as second quartile.

GOODS-ALMA: optically-dark ALMA galaxies shed light on a cluster in formation at z = 3.5

Thanks to its outstanding angular resolution, the Atacama Large Millimeter/submillimeter Array (ALMA) has recently unambiguously identified a population of optically dark galaxies with redshifts greater than z = 3, which play an important role in the cosmic star formation in massive galaxies.

In this chapter, I study the properties of the six optically dark galaxies detected in the 69 arcmin 2 GOODS-ALMA 1.1mm continuum survey. While none of them are listed in the deepest H-band based CANDELS catalog in the GOODS-South field down to H = 28.16 AB, we were able to de-blend two of them from their bright neighbor and measure an H-band flux for them. I present the spectroscopic scan follow-up of five of the six sources with ALMA band 4. All are detected in the 2 mm continuum with signal-to-noise ratios higher than eight. One emission line is detected in AGS4 (‹ obs = 151.44 GHz with a S/N = 8.58) and AGS17 (‹ obs = 154.78 GHz with a S/N = 10.23), which we interpret in both cases as being due to the CO line at z AGS4 spec = 3.556 and z AGS17 spec = 3.467, respectively. These redshifts match both the probability distribution of the photometric redshifts derived from the UV to near-infrared spectral energy distributions (SEDs) and the far-infrared SEDs for typical dust temperatures of galaxies at these redshifts. We note that even though our galaxies are extremely faint at H band and are always blended with the neighbouring galaxies in the images at longer wavelengths, we manage to extract the photometries with the de-blending procedure and put constraints on the UV to near-infrared SEDs. We present evidence that nearly 70 % (4/6 of galaxies) of the optically dark galaxies belong to the same overdensity of galaxies at z ≥ 3.5. The most massive one, AGS24 (M ı = 10 11.32 +0.02 ≠0.19 M § ), is the most massive galaxy without an active galactic nucleus (AGN) at z > 3 in the GOODS-ALMA field. It falls in the very center of the peak of the galaxy surface density, which suggests that the surrounding overdensity is a proto-cluster in the process of virialization and that AGS24 is the candidate progenitor of the future brightest cluster galaxy (BCG).

Introduction

The discovery of bright submm-selected galaxies (SMGs; S(850 µm) > 5mJy; e.g., [START_REF] Blain | Submillimeter galaxies[END_REF] opened up a window on the formation of the most intense episodes of star-formation in massive distant galaxies. It is now widely accepted that understanding the nature of SMGs is a prerequisite to understand the formation of local massive ellipticals and Brightest Cluster Galaxies (BCGs) at the center of the most massive virialized structures. Defining starbursts as galaxies above the star-formation main sequence (MS, star formation rate vs. stellar mass, SFR -M ı correlation; [START_REF] Noeske | Star Formation in AEGIS Field Galaxies since z=1.1: The Dominance of Gradually Declining Star Formation, and the Main Sequence of Star-forming Galaxies[END_REF][START_REF] Elbaz | The reversal of the star formation-density relation in the distant universe[END_REF]; [START_REF] Elbaz | GOODS-Herschel: an infrared main sequence for star-forming galaxies[END_REF]; [START_REF] Daddi | Multiwavelength Study of Massive Galaxies at z ~2. I. Star Formation and Galaxy Growth[END_REF]; [START_REF] Rodighiero | The Lesser Role of Starbursts in Star Formation at z = 2[END_REF], it has been found that some SMGs fall in the starburst category while some are MS galaxies (massive MS galaxies at z = 3 -5 exhibit very high SFR, [START_REF] Schreiber | The Herschel view of the dominant mode of galaxy growth from z = 4 to the present day[END_REF]), some exhibit compact others extended star-formation (see e.g., Hodge19; [START_REF] Elbaz | Starbursts in and out of the star-formation main sequence[END_REF]; [START_REF] Rujopakarn | VLA and ALMA Imaging of Intense Galaxy-wide Star Formation in z 2 Galaxies[END_REF]; [START_REF] Rujopakarn | ALMA 200 pc Resolution Imaging of Smooth Cold Dusty Disks in Typical z 3 Star-forming Galaxies[END_REF]).

Using ALMA to follow-up 63 optically-dark galaxies located in the three Southern hemisphere CANDELS fields (totalizing 600 arcmin 2 in the COSMOS, EGS and GOODS-South fields), [START_REF] Wang | A dominant population of optically invisible massive galaxies in the early Universe[END_REF] showed that these galaxies contribute ten times more than equivalently massive ultraviolet-bright galaxies to the cosmic star-formation rate density at z > 3. Despite being bright -although 3 to 10 times fainter than the classical SMG population -in the sub-millimeter (here 870 µm), these galaxies unseen in the deepest UV to near-infrared surveys were selected as H-dropouts with an IRAC counterpart brighter than 24 AB at 4.5 µm. Thanks to the angular resolution of ALMA (0 ÕÕ .6 here), it is indisputable that these galaxies exhibit no counterpart from the UV to the H-band. While in the observations from single dish telescopes such as SCUBA,LABOCA, and the IRAM 30 m, LMT, several optical sources can play the role of candidate counterparts due to the large beam size. It took 14 years [START_REF] Walter | The intense starburst HDF 850.1 in a galaxy overdensity at z ¥ 5.2 in the Hubble Deep Field[END_REF] to identify the galaxy responsible for the brightest Submillimetre Common-User Bolometer Array (SCUBA) source in the HUDF (Hughes et al., 1998, HDF850.1). The H-dropout (or optically-dark) galaxies followed-up by ALMA experience lower, hence more normal, star-formation activity than the more classical SMGs such as those followed-up by ALMA as part of the ALESS project (see e.g. da [START_REF] Da Cunha | An ALMA Survey of Sub-millimeter Galaxies in the Extended Chandra Deep Field South: Physical Properties Derived from Ultraviolet-to-radio Modeling[END_REF], [START_REF] Swinbank | An ALMA survey of sub-millimetre Galaxies in the Extended Chandra Deep Field South: the far-infrared properties of SMGs[END_REF]). The ALMA detected ones exhibit <SFR> = 200 M § yr ≠1 typical of MS galaxies at these redshifts although with a large scatter, whereas the undetected ones experience much lower SFR [START_REF] Wang | A dominant population of optically invisible massive galaxies in the early Universe[END_REF].

The clustering properties of these H-dropout galaxies -derived from their cross-correlation with H-detected galaxies with similar redshifts and masses -shows that they inhabit massive dark matter haloes (M h ≥10 13±0.3 h ≠1 M § at z = 4) and constitute excellent candidate progenitors of the most massive galaxies presently located in the center of massive groups and galaxy clusters [START_REF] Wang | A dominant population of optically invisible massive galaxies in the early Universe[END_REF]. Recent simulations strengthen this hypothesis by showing that galaxies in (proto)clusters at z > 3 may produce as much as 50 % of the cosmic SFR density [START_REF] Chiang | Galaxy Protoclusters as Drivers of Cosmic Star Formation History in the First 2 Gyr[END_REF]. Numerical simulations can be used to trace back the positions of the galaxies that will end up in a present-day galaxy cluster. [START_REF] Chiang | Ancient Light from Young Cosmic Cities: Physical and Observational Signatures of Galaxy Proto-clusters[END_REF] followed-up these galaxies in the stage preceding virialization, which they called the proto-cluster phase. They defined a useful reference effective radius encompassing 65% of the mass that will end up in the cluster, i.e. 65% of the proto-cluster mass as well. At a redshift of z ≥ 3, that will be of interest in our analysis, this radius ranges from 5 comoving-Mpc (hereafter cMpc) for a M(z = 0) = 3 ◊ 10 14 M § bound halo to > 8 cMpc for a Coma-like M(z = 0) = 10 15 M § halo. This is in line with the enclosed sizes, which contain 90% of the stellar mass in proto-clusters, found in [START_REF] Muldrew | What are protoclusters? -Defining high-redshift galaxy clusters and protoclusters[END_REF]. Identifying a proto-cluster from observations is much more complex since it is difficult to determine the future of any given over-density of galaxies. However, when studying galaxy over-densities the effective radius defined from numerical simulations may be used as a reference size for a potential proto-cluster. Based on the standard hierarchical models of galaxy formation, numerical simulations demonstrate that massive galaxies at high redshifts are useful tracers of over-dense regions since they preferentially form within the peaks in the density field [START_REF] Springel | Simulations of the formation, evolution and clustering of galaxies and quasars[END_REF][START_REF] De Lucia | The hierarchical formation of the brightest cluster galaxies[END_REF]. At the same time, recent ALMA surveys have revealed that ALMA is a powerful tool in detecting massive galaxies at high redshift [START_REF] Dunlop | A deep ALMA image of the Hubble Ultra Deep Field[END_REF][START_REF] Hatsukade | ALMA twenty-six arcmin 2 survey of GOODS-S at one millimeter (ASAGAO): Source catalog and number counts[END_REF][START_REF] Franco | GOODS-ALMA: 1.1 mm galaxy survey. I. Source catalog and optically dark galaxies[END_REF] taking advantage of its high sensitivity and high spatial-resolution. As a result, not only can such a population represent a unique probe of star-formation at cosmic epochs when dust attenuation is largely unknown, i.e., at z > 3, but it can also be seen as potential beacons to identify clusters at the epoch of their formation. Studying the connection of optically-dark ALMA galaxies with the formation of large-scale structures is the main goal of the present paper.

The study on how galaxy clusters form and evolve across cosmic time is critical to test models of large-scale structure formation. The formation of galaxy clusters is related to the collapse of the gravitationally bound over-densities originating from the peaks in the initial density fluctuations since the Big Bang [START_REF] Kravtsov | Formation of Galaxy Clusters[END_REF]. It is also well-known that galaxies evolve differently in different environments at early times: galaxies in denser regions tend to form earlier and evolve faster, as indicated by their ages and enhanced SFR, than galaxies in less dense regions [START_REF] Thomas | The Epochs of Early-Type Galaxy Formation as a Function of Environment[END_REF][START_REF] Elbaz | The reversal of the star formation-density relation in the distant universe[END_REF][START_REF] Gobat | Star formation histories of early-type galaxies at z = 1.2 in cluster and field environments[END_REF]. Therefore, by exploring the assembly of the most massive dark matter halos we hope to better understand how massive galaxies formed, in the framework of large-scale structure formation.

Various techniques have been developed to search for high-redshift galaxy clusters. However, a complete search for galaxy clusters at high redshift (&2) becomes more difficult as the cluster properties evolve dramatically at an early age. Some galaxy clusters are mature systems with evidence of quiescent galaxies dominating the center [START_REF] Papovich | CANDELS Observations of the Structural Properties of Cluster Galaxies at z = 1.62[END_REF][START_REF] Newman | Spectroscopic Confirmation of the Rich z = 1.80 Galaxy Cluster JKCS 041 using the WFC3 Grism: Environmental Trends in the Ages and Structure of Quiescent Galaxies[END_REF][START_REF] Willis | Spectroscopic confirmation of a mature galaxy cluster at a redshift of 2[END_REF], while others still show substantial star formation activities in the center [START_REF] Brodwin | The Era of Star Formation in Galaxy Clusters[END_REF][START_REF] Webb | An Extreme Starburst in the Core of a Rich Galaxy Cluster at z = 1.7[END_REF]Wang et al., 2016a). As a result, infrared/submilimeter (infrared/submm) surveys can be beneficial in revealing the over-densities [START_REF] Daddi | Two Bright Submillimeter Galaxies in a z = 4.05 Protocluster in Goods-North, and Accurate Radio-Infrared Photometric Redshifts[END_REF][START_REF] Capak | A massive protocluster of galaxies at a redshift of z˜5.3[END_REF][START_REF] Miller | A massive core for a cluster of galaxies at a redshift of 4.3[END_REF][START_REF] Oteo | An Extreme Protocluster of Luminous Dusty Starbursts in the Early Universe[END_REF][START_REF] Gómez-Guijarro | Confirming Herschel Candidate Protoclusters from ALMA/VLA CO Observations[END_REF][START_REF] Castignani | Environmental processing in cluster core galaxies at z = 1.7[END_REF], and extensive in-depth deep surveys from Atacama Large Millimeter Array (ALMA) can make significant contributions.

In this paper, we present the physical properties of six optically-dark galaxies detected in the GOODS-ALMA field based on the new ALMA data obtained from the spectroscopic follow-up of five of these galaxies. We also discuss the connection between their optically dark nature and the dense environment they reside in.

The GOODS-ALMA survey makes use of the 69 arcmin 2 ALMA image covering the deepest region of the Great Observatories Origins Deep Survey South (GOODS-South) field. In this large blind survey, massive (>10 10.5 M § ) and dust-rich galaxies at high redshift are efficiently detected due to their low surface number density in the early Universe. For more details of this survey we refer the readers to [START_REF] Franco | GOODS-ALMA: 1.1 mm galaxy survey. I. Source catalog and optically dark galaxies[END_REF], hereafter F18).

We use cosmological parameters of H 0 = 70 km s ≠1 Mpc ≠1 , M = 0.3, and = 0.7. A Salpeter (1955) initial mass function (IMF) is adopted to derive stellar masses and SFRs. We convert values obtained by other studies from a [START_REF] Chabrier | Galactic Stellar and Substellar Initial Mass Function[END_REF] IMF to a [START_REF] Salpeter | The Luminosity Function and Stellar Evolution[END_REF] IMF by multiplying SFR and stellar masses by the same factor 1.74.

Data and observations

3.2 Data and observations 3.2.1 ALMA data and observations

GOODS-ALMA survey data

We use data from GOODS-ALMA, a 1.1mm cosmological survey of 69 arcmin 2 (69◊10 Õ ) in the GOODS-South field (program 2015.1.00543.S, PI: D.Elbaz). GOODS-ALMA reaches an rms sensitivity of ‡ ƒ 0.18 mJy beam ≠1 in the 0 ÕÕ .60 tapered mosaic (F18). The comparison between the redshift distribution of the detections in the GOODS-ALMA survey (F18; F20a) and the one in the smaller pencil beam ALMA survey of the Hubble Ultra Deep Field (4.5 arcmin 2 , Dunlop et al. ( 2017)) shows that, for the same total observing time of about 20 hours, the shallower but 15 times wider GOODS-ALMA survey enables the detection of systematically more distant galaxies (F18; F20a). This is a natural consequence of the fact that high-redshift dusty galaxies are found to be massive. The combination of the low surface density and brightness of these galaxies then favors shallower and wider surveys. A total of 35 galaxies have been detected above 3.5 ‡ in GOODS-ALMA [START_REF] Zhou | GOODS-ALMA: Optically dark ALMA galaxies shed light on a cluster in formation at z = 3.5[END_REF], hereafter F20a). These include 19 galaxies detected above the 4.8 ‡ limit from a blind detection approach (F18) and 16 galaxies within 3.5 and 4.8 ‡ detected using ancillary information, mainly Spitzer -IRAC prior positions (F20a). The median redshift and stellar mass of the S/N Ø 4.8 sources are z = 2.73 and M ı = 1.0 ◊ 10 11 M § , whereas the sources with a 4.8 > S/N Ø 3.5, are both slightly closer (z = 2.40) and less massive (M ı = 7.2 ◊ 10 10 M § ).

ALMA follow-up observations of five optically-dark galaxies

Out of the 35 ALMA galaxies presented in Section 3.2.1.1, an ensemble of six galaxies are opticallydark, or also called HST-dark since they are not detected in the optical to near-infrared HST images down to a 5 ‡ H-band detection limit (HST-WFC3/F160W) of H = 28.16 AB mag [START_REF] Guo | CANDELS Multi-wavelength Catalogs: Source Detection and Photometry in the GOODS-South Field[END_REF]. (12.5 %; F20a), for a global fraction of 17 % of optically dark galaxies. As a result, about 17% of the sources detected above 3.5 ‡ in the GOODS-ALMA blind survey are optically dark (and 21% above 4.8 ‡). Out of these six sources initially classified as optically dark, we will show in Section 3.4.1 and 3.5.3.2 that two are associated with an H-band counterpart. They were missed from the H-band catalog due to blending.

We have obtained ALMA spectroscopic follow-up data for five of these optically-dark galaxies (AGS4, 11, 15, 17, 24) that we discuss in Section 3.4. The sixth one, AGS25, was identified after the ALMA call hence no follow-up data have been obtained for this one.

We followed-up these five optically-dark sources with ALMA band 4 using the spectroscopic scan mode in Cycle 7 (project 2018.1.01079.S PI: M. Franco). The band 4 observations were performed by combining four tunings to cover a total frequency range of 137.2-165.8 GHz that ensures a broad redshift coverage from z = 2 to z = 5, with at least one line detected among the possible CO transitions from J = 4 to J = 7, CI and H 2 O. The observations were carried out with the array configuration C43-4, with a synthesized beam of ≥0 ÕÕ .88. Each source was observed for 13 min on-source time in each tuning, reaching a typical rms sensitivity of 0.2 mJy per 400 km s ≠1 . We reduced the raw data using the standard ALMA pipeline with Common Astronomy Software Application package (CASA, McMullin et al., 2007). Then we converted the data into uvfits format and performed analysis with the IRAM GILDAS tool (Gildas Team, 2013) using the same method as in [START_REF] Jin | Discovery of Four Apparently Cold Dusty Galaxies at z=3.62 5.85 in the COSMOS Field: Direct Evidence of Cosmic Microwave Background Impact on High-redshift Galaxy Observables[END_REF].

Ancillary data

GOODS-ALMA benefits from abundant ancillary data. We summarize in the following Sections the data that we use in the present study. For more details we refer the reader to Franco et al. (2018, Section 2.4.).

Optical/near-infrared data

We use the data from the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS; [START_REF] Grogin | CANDELS: The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey[END_REF]) obtained using images from the Wide Field Camera 3 (WFC3) and the Advanced Camera for Surveys (ACS) on-board the Hubble Space Telescope (HST) in the filters: Y 125 , J 125 , B 435 , V 606 , i 775 , i 814 and z 850 .

We also use images from the VLT, obtained in the U -band with VIMOS [START_REF] Nonino | Deep U Band and R Imaging of Goods-South: Observations, Data Reduction and First Results[END_REF], and in the K s -band with ISAAC [START_REF] Retzlaff | The Great Observatories Origins Deep Survey. VLT/ISAAC near-infrared imaging of the GOODS-South field[END_REF] and HAWK-I [START_REF] Fontana | The Hawk-I UDS and GOODS Survey (HUGS): Survey design and deep K-band number counts[END_REF] and the associated CANDELS multiwavelength catalog [START_REF] Guo | CANDELS Multi-wavelength Catalogs: Source Detection and Photometry in the GOODS-South Field[END_REF]. In the near-infrared, we also use images and data from the FourStar galaxy evolution survey (ZFOURGE, PI: I. Labbé; [START_REF] Straatman | The FourStar Galaxy Evolution Survey (ZFOURGE): Ultraviolet to Far-infrared Catalogs, Medium-bandwidth Photometric Redshifts with Improved Accuracy, Stellar Masses, and Confirmation of Quiescent Galaxies to z 3.5[END_REF]). These include narrow-band filter images in the J (J 1 , J 2 , J 3 ) and H (H s , H l ) bands useful for the photometric redshift determination. The ZFOURGE team produced the deepest K s -band detection image by combining images from the survey itself together with images from previous surveys and reaching a 5 ‡ limiting depth of 26.2 ≥ 26.5 AB mag in the whole Chandra Deep Field South (Giacconi et al., 2002, CDFS).

Mid-infrared data

We use Spitzer Infrared Array Camera (IRAC) images at 3.6 and 4.5 µm, reaching a depth of 26.5 AB mag, from the ultradeep mosaics of the IRAC Ultra Deep Field (Labbé et al., 2015, IUDF). These were obtained by combining the observations from the IUDF (PI: Labbé) and IRAC Legacy over GOODS (IGOODS, PI: Oesch) programs, together with archival data from GOODS (PI: Dickinson), S-CANDELS (PI: Fazio), ERS (PI: Fazio), and UDF2 (PI: Bouwens).

Radio data

We use radio images at 3 GHz and 6 GHz from the Karl G. Jansky Very Large Array (VLA; PI: W. Rujopakarn) which cover the entire GOODS-ALMA field. The 6 GHz map has an angular resolution of 0 ÕÕ .31 ◊ 0 ÕÕ .61. The exposure time is in total 177 hours reaching an rms noise at the pointing center of 0.32 µJy beam ≠1 . The 3 GHz map has an angular resolution of 1 ÕÕ .19 ◊ 0 ÕÕ .59 and an average rms noise of 1.03 µJy beam ≠1 . At 3 GHz, AGS4, 17 and 24 are detected with a S/N of 10.4, 9.9, and 5.6 using PyBDSF1 (Rujopakarn et al. in prep.). We examined the image and noticed that the peak fluxes of AGS11, 15 and 25 are at 3 < S/N < 5. At 6 GHz, only AGS4 is detected with S/N > 5 using PyBDSF 1 , AGS17, 24 and 25 show peak fluxes at at 3 < S/N < 5 in the image. We list the fluxes in Table 3.1 .

X-ray data

We use the catalog of [START_REF] Luo | The Chandra Deep Field-South Survey: 7 Ms Source Catalogs[END_REF] obtained from the 7 Msec X-ray data in the CDFS field observed in three bands: 0.5-7.0 keV, 0.5-2.0 keV, and 2-7 keV. This catalog includes a classification of candidate X-ray active galactic nuclei (AGN) that we discuss in the following of the paper.

Origin of the redshifts and stellar masses

In total, 11621 galaxies located in the GOODS-ALMA field have been attributed either a photometric redshift (92 % of the sample) or a spectroscopic redshift (894 galaxies). We discuss below the origin of these redshifts, including the newly obtained ones from the VANDELS survey that we use to improve the contrast on the z≥3.5 structure that we will discuss in the following sections.

Photometric redshifts and stellar masses

The photometric redshifts used in the present study come from the catalog generated by the ZFOURGE team [START_REF] Straatman | The FourStar Galaxy Evolution Survey (ZFOURGE): Ultraviolet to Far-infrared Catalogs, Medium-bandwidth Photometric Redshifts with Improved Accuracy, Stellar Masses, and Confirmation of Quiescent Galaxies to z 3.5[END_REF][START_REF] Forrest | Discovery of Extreme [O III]+H-Emitting Galaxies Tracing an Overdensity at z 3.5 in CDF-South[END_REF]. We use the redshift maximizing the likelihood (minimized ‰2 ), z peak , as derived by fitting the observed SEDs with the code EAzY [START_REF] Brammer | EAZY: A Fast, Public Photometric Redshift Code[END_REF]. These photometric redshifts present an excellent agreement with existing spectroscopic redshifts with ‡ z = 0.020 at z > 1.5 [START_REF] Straatman | The FourStar Galaxy Evolution Survey (ZFOURGE): Ultraviolet to Far-infrared Catalogs, Medium-bandwidth Photometric Redshifts with Improved Accuracy, Stellar Masses, and Confirmation of Quiescent Galaxies to z 3.5[END_REF], where ‡ z is the normalized median absolute deviation (NMAD) of z/(1 + z), i.e., 1.48 ◊ the median absolute deviation (MAD) of |z photz spec |/(1 + z spec ).

We adopt the stellar masses of the ZFOURGE catalog which were derived with [START_REF] Bruzual | Stellar population synthesis at the resolution of 2003[END_REF] stellar population synthesis models [START_REF] Straatman | The FourStar Galaxy Evolution Survey (ZFOURGE): Ultraviolet to Far-infrared Catalogs, Medium-bandwidth Photometric Redshifts with Improved Accuracy, Stellar Masses, and Confirmation of Quiescent Galaxies to z 3.5[END_REF] assuming exponentially declining star formation histories (SFH) and a [START_REF] Calzetti | The Dust Content and Opacity of Actively Star-forming Galaxies[END_REF] dust attenuation law.

Spectroscopic redshifts

The spectroscopic redshifts are both from the literature, as compiled in the ZFOURGE catalog, and from the latest data release from the VANDELS survey (DR3 2 ). VANDELS targets star-forming galaxies at 5.5 Ø z Ø 2.5 and massive passive galaxies at 2.5 Ø z Ø 1.0 in the CANDELS CDFS and UDS fields. It is an ultra-deep spectroscopic survey of high-redshift galaxies with exposure times ranging from 20 to 80 hours per source [START_REF] Pentericci | The VANDELS ESO public spectroscopic survey: Observations and first data release[END_REF][START_REF] Mclure | The VANDELS ESO public spectroscopic survey[END_REF]. In total, we obtain 132 new spectroscopic redshifts, z spec , in the GOODS-ALMA field from VANDELS and 83 of them fall in the redshift range 4 Ø z Ø 3.

Derived parameters of the optically dark galaxies

Redshifts

To derive the photometric redshifts, we first de-blend3 AGS4, 15, and 24 from their neighbors in the observed optical to mid-infrared images (ancillary data in Section 3.2.2), as to be discussed in detail in Section 3.4. The photometric data are then used to measure the redshifts by fitting the optical to mid-infrared (rest-frame UV to near-infrared) SEDs with EAzY [START_REF] Brammer | EAZY: A Fast, Public Photometric Redshift Code[END_REF]. The photometric redshift of AGS25 comes from its K s -band counterpart in the ZFOURGE catalog, ID ZFOURGE = 11353. AGS11 and 17 are either too faint or too close to a bright emitter in the images to be de-blended properly and derive meaningful redshifts, therefore we do not have photometric redshifts for these two galaxies.

The spectroscopic redshifts of AGS4 and AGS17 are obtained based on the detected emission lines in the ALMA spectroscopic follow-up and their SEDs at UV to near-infrared and far-infrared (rest-frame), which will be discussed in detail in Section 3.3.

Stellar masses

The stellar masses of AGS4, 15 and 24 are derived from the observed optical to mid-infrared SED fitted with FAST++4 assuming a delayed exponentially declining SFH and a [START_REF] Calzetti | The Dust Content and Opacity of Actively Star-forming Galaxies[END_REF] attenuation law. The stellar masses of AGS11 and 17 are calculated based only on the IRAC emission. The stellar mass of AGS25 comes from its K s -band counterpart in the ZFOURGE catalog.

Infrared luminosities

Infrared luminosities are measured by the SED fitting of the Herschel and ALMA flux densities for AGS4 and AGS17 using CIGALE [START_REF] Boquien | CIGALE: a python Code Investigating GALaxy Emission[END_REF] as in F20b. The other four galaxies are not detected by Herschel, then their infrared luminosity was derived by fitting the ALMA 1.1mm point with the SED templates in Schreiber et al. (2018), as described in F20b.

SFRs

The star formation rates are measured in Franco et al. (2020a, hereafter, ), as SFR tot = SFR IR + SFR UV . SFR IR is calculated based on the correlation between infrared luminosity and SFR in Kennicutt (1998b) and SFR UV contributes only 1% to SFR in the GOODS-ALMA detections (F20b), therefore it is negligible.

All six galaxies are detected in the radio at 3GHz with flux densities ranging from 4 µJy to 40 µJy (Table . 3.1). This allows us to estimate a radio SFR. With such extremely faint flux densities, they are not expected to present a radio excess that could be attributed to an AGN, but we checked this as follows. We calculated the luminosity at 1.4GHz (restframe) from the flux at 3GHz assuming a radio spectral index of -= -3GHz 1.4GHz = ≠0.8, the typical value for galaxies at z > 2 [START_REF] Delhaize | The VLA-COSMOS 3 GHz Large Project: The infraredradio correlation of star-forming galaxies and AGN to z 6[END_REF]:

L 1.4 GHz (W Hz ≠1 ) = 4fiD 2 L (1 + z) -+1 3 1.4 3 4 - S 3GHz . (3.1)
where D L is the luminosity distance to the object. The results are listed in Table 3.1. Then we compared the ratio L 1.4GHz /SFR with the radio excess threshold as in [START_REF] Delvecchio | The VLA-COSMOS 3 GHz Large Project: AGN and host-galaxy properties out to z . 6[END_REF]. All the six optically dark galaxies have L 1.4GHz /SFR well below threshold, with the one of AGS24 being five time lower and the others at least ten times lower. This is fully consistent with star-forming galaxies at z ≥ 3.5. A radio AGN contribution cannot be ruled out, but it must be contributing for a very small fraction to the total radio emission. Nevertheless, due to vigorous star formation dominating the total radio emission at low resolution, only Very Long Baseline Interferometry (VLBI) observations could unambiguously shed light on the possible presence of radio AGN activity on circum-nuclear (< 100pc) scales.

We also checked in the CDFS 7Ms catalog [START_REF] Luo | The Chandra Deep Field-South Survey: 7 Ms Source Catalogs[END_REF] and found that an X-ray source is located equally close to AGS4 and another galaxy, ID ZFOURGE = 12333. Therefore, it is not clear whether this X-ray source is associated with our optically dark source, AGS4. A detailed study of the AGN contamination to the SFR of AGS4 is presented in F20b where it is found to be negligible.

Results of the ALMA spectroscopic follow-up

In this section, we present the results of the ALMA spectroscopic follow-up. As introduced in Section 3.2.1.2, five of the six optically dark galaxies are followed up, as AGS25 was identified after the ALMA call. One emission line is detected in AGS4 and AGS17, and we constrained the upper limits for AGS11, 15 and 24.

AGS4

The spectrum of AGS4 shows a clear line detection with a S/N = 8.58 at ‹ obs = 151.44 GHz (see Fig. 3.1-Left). One ALMA spectral line alone is not sufficient to obtain a definitive spectroscopic redshift for AGS4. However, out of the various possible lines that may be responsible for the one that is detected, only three lines can be reconciled with the 4000 Å break that is seen in the optical to near-infrared SED of AGS4 (Fig. 3.3-Top-left, see more discussion on the SED in Section 3.4.1), namely the CO , CO (7-6) andH 2 O(2 11 -2 02 ) lines. We exclude H 2 O(2 11 -2 02 ) since it is typically less bright or as bright [START_REF] Yang | Submillimeter H2O and H2O + emission in lensed ultra-and hyperluminous infrared galaxies at z 2-4[END_REF], flux ratio from 0.4 to 1.1) as the neighboring high-J CO transition lines (7-6 or 6-5) and we do not find any evidence for a second line in the ALMA spectrum that can be comparable to the 8.7 ‡ detection.

The CO(7-6) line with a rest-frame frequency of ‹ RF = 806.7 GHz provides a spectroscopic redshift of z AGS4 spec = 4.326 which agrees with one of the two peaks of the probability distribution function (PDF) of the optical photometric redshift. We note that the [CI](2-1) (‹ rf = 809.3 GHz) line falls very close to the CO(7-6) line and is not detected here with a 3.5 ‡ limit of 0.7 mJy which corresponds to a [CI](2-1)/CO(7-6) ratio of 0.40. This flux ratio falls at the lower end of the trend defined by the MS galaxies, starburst galaxies and AGNs in the local universe and at high-z (data compiled and described in [START_REF] Valentino | A Survey of Atomic Carbon [C I] in High-redshift Main-sequence Galaxies[END_REF]Valentino et al., 2020a), as shown in Fig. 3.2. Chance is small to have AGS4 to be an extreme case of the [CI](2-1)/CO(7-6) flux ratio. The CO line with a rest-frame frequency of ‹ RF = 806.7 GHz gives a z AGS4 spec = 3.566 which agrees with the 4000 Å discontinuity in the optical SED. Therefore, we adopt the redshift to be z AGS4 spec = 3.556 assuming the line to be CO .

AGS17

Our spectroscopic scan follow-up of AGS17 shows an emission line at 154.78 GHz with a S/N = 10.23. The far-infrared SED in this region peaks at around 400 µm (F18, Fig. 22). Taking this far-infrared SED into account, the detected line is likely to be CO or CO or CO(7-6) since the higher-J or lower-J transitions give redshifts leading to unrealistic dust temperatures with dust emission peaking at ⁄ < 67 µm or at ⁄ > 134 µm, restframe. We checked the three L IR -L CO relation for the three transitions. We found that the ALMA flux of AGS17 and the corresponding L IR for the three different redshifts, which are z = 2.723, 3.467, 4.212, will bring the galaxy to the positions which are consistent with the L IR -L CO relations of the three transitions [START_REF] Lisenfeld | Sub)millimetre emission from NGC 1569: An abundance of very small grains[END_REF]. Therefore, we cannot disentangle these possibilities based on the single line.

However, if we assume the line to correspond to the CO(7-6) transition (‹ rf = 806.7 GHz), a second line, [CI] (‹ rf = 809.3 GHz), is supposed to be detected at similar significance (Valentino et al., 2020a) and it is not the case for AGS17. We are then left with two possibilities: a spectroscopic redshift of z AGS17 spec = 2.723 in the case of the CO(5-4) transition and the other of z AGS17 spec = 3.467 for the CO(6-5) transition. The second redshift of z AGS17 spec = 3.467 turns out to fall within a redshift peak in the redshift distribution of the galaxies in the field which also exhibits a concentration of galaxies right around AGS17, as we will discuss in more details in Section 3.5.3. For that reason, despite the fact that we cannot strictly rule out the possibility that we are here observing the CO(5-4) transition, we decided to favor the CO transition from a purely probabilistic point of view. A mild confirmation of this choice comes from the dust temperature that we obtain when assuming one or the other transition and redshift. If the transition is CO , the spectroscopic redshift z AGS17 spec = 2.723 implies a dust temperature T dust ≥ 30 K, using a simple conversion of the peak of the SED at around 400 µm to the dust temperature. This is lower than the typical temperature T dust ≥ 38 K for the main sequence galaxies at this redshift [START_REF] Schreiber | Dust temperature and mid-to-total infrared color distributions for star-forming galaxies at 0 z 4[END_REF]. Instead, if we assume the transition to be CO , the z AGS17 spec = 3.467 indicates a dust temperature T dust ≥ 37 K which is close to the typical dust temperature T dust ≥ 40 K at z = 3.5 [START_REF] Schreiber | Dust temperature and mid-to-total infrared color distributions for star-forming galaxies at 0 z 4[END_REF] and is consistent with the mean dust temperature, T dust = 37 K, of the 39 optically dark galaxies in [START_REF] Wang | A dominant population of optically invisible massive galaxies in the early Universe[END_REF]. We note that we also tried to obtain an independent confirmation of the redshift of AGS17 from its UV to near-IR SED as we did for AGS4. However, as we will discuss in Section 3.5.3.2 and as shown in Fig. 3.8-Bottom, the optical emission of AGS17 suffers from a strong blending with bright neighbors that prevents us from extracting a meaningful SED that we could use to obtain a robust, or even tentative, model fit and photometric redshift derivation.

Upper limits of AGS11, AGS15 and AGS24

Our spectroscopic scan follow-up of AGS11, AGS15 and AGS24 does not show emission line detections higher than 5 ‡. Based on their assumed redshifts, as will be discussed in Section 3.5.3, the ALMA spectroscopic scans encompass the position of the CO emission line in the three galaxies. At the assumed redshifts, the infrared luminosities can be converted to CO luminosities using the relation defined by [START_REF] Lisenfeld | Sub)millimetre emission from NGC 1569: An abundance of very small grains[END_REF] and then to CO fluxes accordingly. We find that the non-detection of the CO(6-5) line agrees with the dispersion of the L IR -L Õ CO 6≠5 relation with values below 3 ‡ for the three galaxies (2.8 ‡, 2.3 ‡, 0.7 ‡ for AGS11, AGS15 and AGS24 respectively).

GOODS-ALMA optically-dark galaxies

As described in Section 3.2.1.1, six out of the 35 GOODS-ALMA detections do not have counterparts in the CANDELS catalog [START_REF] Guo | CANDELS Multi-wavelength Catalogs: Source Detection and Photometry in the GOODS-South Field[END_REF]. Despite their non detection in the H-band images, we are able to measure the residual emission in the optical to near-infrared images after subtracting the contribution of neighboring galaxies and to use the resulting measurements to obtain photometric redshift determinations as discussed in the next sections. After a careful analysis of the H-band images of these galaxies, we realized that in two cases, AGS4 and AGS15, there was a clear association in the H-band image that was matching the ALMA contours. In both cases, the H-band detection was close to a bright H-band neighbor and both objects were interpreted in the CANDELS catalog as a 3.4 GOODS-ALMA optically-dark galaxies single object, hence neglecting the ALMA source. If we were to extrapolate our small number statistics to the population of optically-dark galaxies at these depths, this would imply that about 35 % of the optically-dark sources are mistaken as such due to confusion. We note however, that these galaxies are detected here because of the very deep H-band image since their AB magnitudes are H = 25.23 and 27.11 AB for AGS4 and AGS15, respectively.

All four of the optically-dark galaxies detected from a blind detection above 4.8 ‡ are detected with IRAC and listed in the catalog of [START_REF] Ashby | S-CANDELS: The Spitzer-Cosmic Assembly Near-Infrared Deep Extragalactic Survey. Survey Design, Photometry, and Deep IRAC Source Counts[END_REF]. These galaxies are AGS4, 11, 15 and 17. The remaining two optically-dark galaxies, AGS24 and AGS25, were detected in the ALMA image using IRAC priors in F20a. However, they were not listed in the catalog of [START_REF] Ashby | S-CANDELS: The Spitzer-Cosmic Assembly Near-Infrared Deep Extragalactic Survey. Survey Design, Photometry, and Deep IRAC Source Counts[END_REF]. In both cases, F20a (see also Section 3.5.3.1) noticed that despite being well above the IRAC detection limit, these two galaxies were missed because of the presence of a bright neighbor due to confusion (see Fig. 3.7). After carefully subtracting the surrounding neighbors, the flux densities measured at 3.5 µm for both galaxies are F 3.6µm (AGS24) = 4.7 ± 0.5 µJy and F 3.6µm (AGS25) = 1.5 ± 0.1 µJy.

Five of the GOODS-ALMA optically-dark galaxies (AGS4, 11, 15, 17 and 24) were followed-up with ALMA band 4 (see Section 3.2.1.2) and all are confirmed with a S/N of 14.7, 16.8, 15.3, 15.9 and 8.7 in the 2 mm continuum. AGS25 was not included in the ALMA proposal because the list of sources below the 4.8 ‡ limit that includes it was still under construction at the time. All the six optically dark galaxies are also detected above 4.5 ‡ at 870 µm continuum [START_REF] Cowie | A Submillimeter Perspective on the GOODS Fields (SUPER GOODS). III. A Large Sample of ALMA Sources in the GOODS-S[END_REF]. Hence there is no doubt that they are all real.

Another confirmation of the robustness of four of the optically-dark galaxies -AGS4, 17 and 24-comes from their detection in the radio with the VLA at 3 GHz with a S/N of 10.4, 9.9, and 5.6, respectively. AGS11, 15 and 25 are not detected with S/N higher than 5 (Rujopakarn et al. in prep.). The first two of the detected galaxies, AGS4 and AGS17, are the only optically-dark galaxies in the sample that are also detected with Herschel and their far-infrared SED is well-measured with no less than five Herschel data points from 100 to 500 µm in the observed frame. We discuss in the following two sections the properties of AGS4 and AGS25.

3.4.1 AGS4, an extremely massive galaxy at z=3.556 and a case of blending in the Hubble H-band image AGS4 is detected in GOODS-ALMA with a S/N = 9.7 at 1.1 mm (F18, see also red solid contours in Fig. 3.4). Using the ALMA follow-up observations presented in Section 3.3.1, we now have a confirmation of the robustness of this source through its detection with ALMA at 2 mm with an even higher S/N = 14.7 (green dashed contours in Fig. 3.4).

AGS4 is an illustration of what we may call spectral de-confusion. The two catalogs CANDELS and ZFOURGE have identified a single object associated with this position on the sky. The CANDELS position is close to the center of the bright optical emitter whereas the ZFOURGE position is close to the bright K s -band near-infrared emitter. The position of the ALMA detection exhibits a clear offset with respect to the bright optical emitter and falls on the North-West blob that is the brightest in the near-infrared. As shown in Fig. 3.4, AGS4 is at 0 ÕÕ .5 from a bright optical emitter. They are considered as two regions of the same galaxy (ID CANDELS = 8923, z phot = 0.24, cyan "x" in Fig. 3.4) in the CANDELS catalog [START_REF] Guo | CANDELS Multi-wavelength Catalogs: Source Detection and Photometry in the GOODS-South Field[END_REF]. The ZFOURGE position is at 0 ÕÕ .16 (ID ZFOURGE = 12333) to AGS4 (magenta "x" in Fig. 3.4), and the photometic redshift derived by the ZFOURGE team is z 12333 phot = 3.76.

As discussed above, AGS4 exhibits an H-band counterpart well above the WFC3 detection limit in GOODS-South of H = 25.23 AB that was missed due to blending with a bright neighbor. Therefore, despite being truly optically-dark in the optical bands, AGS4 is not strictly speaking an optically-dark galaxy since it is detected in the H-band. After de-blending 3 both emitters -none of which corresponds to the centroid of the CANDELS or ZFOURGE positions which are both in between the two emitterswe can clearly see two different SEDs (Fig. 3.3-Top-left). We note that the two ALMA detections at 1.1mm and 2mm are well-centered on the North-West emitter. The photometric redshift associated to this object, once deconvolved from the bright optical emitter, is z AGS4 phot = 4.02 +0.36 ≠0.60 . We note that the optical source identified in the CANDELS catalog (ID CANDELS = 8923) is clearly offset from the ALMA contours, leaving little doubt that these are two separate galaxies. It is only in the low resolution K sband image that some confusion takes place. Being near-IR based, the ZFOURGE catalog identified the position of the peak K s emission which matches the ALMA galaxy and attributed to it a photometric redshift of z ZFOURGE phot =3.76 (ID ZFOURGE = 12333). The SED fittings of the photometric points associated to the CANDELS (grey) and ZFOURGE (green) galaxies are both shown in Fig. 3.3-Top-left. The grey SED does not leave any doubt on the fact that this is a very nearby galaxy (z CANDELS phot =0.24), with a clear drop of the emission above 0.8 µm, whereas the green SED exhibits a break around 1.6 µm (z ZFOURGE phot =3.76). The very strong difference between the SEDs of the CANDELS and ZFOURGE sources clearly show that they cannot be considered as two sides of the same galaxy, one visible in the optical the other one in the near to far infrared. These two redshifts suggest that there are indeed two galaxies in projection here.

Our ALMA spectral-scan follow-up presented in Section 3.3.1 provides complementary evidence for this spectral de-confusion between two super-imposed galaxies with a clear line detection that matches the CO transition at a redshift of z spec = 3.566 (see Fig. 3.1-Left), well within the range of possibilities of our photometric redshift estimate. Adopting the redshift at z AGS4 spec = 3.556, the stellar mass of AGS4 is M ı = 10 11.09±0.08 M § which makes AGS4 one of the most massive galaxies in the early Universe. Note that adopting z AGS4 spec = 4.326, as for CO(7-6), the stellar mass that we obtain for AGS4 is M ı = 10 11.45±0.20 M § , which would make AGS4 the most massive galaxies known in the Universe above z = 4.

We investigated the possibility that the stellar mass of AGS4 may be contaminated by the presence of the continuum from an AGN, but we did not find any evidence of a power-law continuum emission in the SED presented in the Fig. 23 of F18 (also in F20b), nor any radio emission associated to it. There is an X-ray detected AGN that is associated with its neighbor (ID ZFOURGE = 12333) which could potentially affect the measurement if attributed to the wrong counterpart, but again we do not see any clear signature of such effect. The q TIR -parameter (see Eq. 3.2) measuring the infrared to radio flux ratio parameter is equal to q TIR = 2.49±0.06, higher than the typical value for star-forming galaxies at z ≥ 3.5 proposed by [START_REF] Delhaize | The VLA-COSMOS 3 GHz Large Project: The infraredradio correlation of star-forming galaxies and AGN to z 6[END_REF], q TIR =2.16 ± 0.06, indicating no radio excess of this galaxy.

q TIR = log 3 L TIR 3.75 ◊ 10 12 Hz 4 ≠ log 3 L 1.4 GHz W Hz ≠1 4 . (3.2)
where L TIR is total infrared luminosity (8 -1000 µm) in unit W, and the L 1.4 GHz = 2.73◊10 24 W Hz ≠1 is derived from the 10.4 ‡ detection of AGS4 at 3 GHz assuming a radio spectral index of -= -3GHz 1.4GHz = ≠0.8, the typical value for galaxies at z > 2 [START_REF] Delhaize | The VLA-COSMOS 3 GHz Large Project: The infraredradio correlation of star-forming galaxies and AGN to z 6[END_REF], using Eq. 3.1.

AGS25, the most distant optically-dark galaxy in GOODS-ALMA

AGS25 is the only optically-dark galaxy in the sample that is identified in the ZFOURGE catalog and is not confused with a neighboring galaxy. The counterpart of AGS25 in the K s -band image is clearly visible in Fig. 3.5 (middle panel). It is associated with the ZFOURGE galaxy ID ZFOURGE = 11353 at z AGS25 phot =4.64. We fitted its photometric measurements as listed in the ZFOURGE catalog using the code EAzY for consistency with the other galaxies, and found a PDF of the photometric redshift well peaked at the redshift given in the ZFOURGE catalog (Fig. 3.3-Bottom-right). We find a peak redshift of z AGS25 phot = 4.71 +0.24 ≠0.24 that encompasses the value from ZFOURGE that we keep for consistency with F20a. This makes AGS25 the most distant optically-dark galaxy in GOODS-ALMA. The IRAC emission of the galaxy is polluted by the contribution of a bright neighbor at a distance of 3 arcsec (outside of the postage stamps shown in Fig. 3.5). This explains why the galaxy is not listed in the IRAC S-CANDELS catalog [START_REF] Ashby | S-CANDELS: The Spitzer-Cosmic Assembly Near-Infrared Deep Extragalactic Survey. Survey Design, Photometry, and Deep IRAC Source Counts[END_REF]. After de-convolution of the bright neighbor, a clear IRAC detection is obtained (see Fig. 3.5 right panel) that was used to determine its flux density in F20a. The stellar mass of AGS25 at this redshift is M ı = 10 10.39 +0.12 ≠0.29 M § (F20a). AGS25 is not detected by Herschel, hence its SFR = 839 ±131 M § yr ≠1 was derived in F20a by adjusting the SED of Schreiber et al. (2018) to the 1.1mm ALMA flux density corresponding to 195 µm in the rest-frame, which is not far from the peak of the far-infrared SED. 3.5 An over-density at z ≥ 3.5 in GOODS-ALMA

Clustering properties of optically dark galaxies

A large statistical sample of IRAC detected optically-dark galaxies in the CANDELS fields studied by [START_REF] Wang | A dominant population of optically invisible massive galaxies in the early Universe[END_REF] shows that the optically-dark galaxies are the most massive galaxies in the early Universe and are probably the progenitors of the most massive galaxies we see in the groups or clusters of galaxy in the local Universe.

Here in the 69 arcmin 2 GOODS-ALMA field, four of the six optically dark galaxies reside in a small region of 5 arcmin 2 only. We calculate that the probability to have four out six galaxies randomly falling in such a small area is only 0.4% from four million Monte Carlo mock realizations. In addition, all the four optically-dark galaxies present a redshift consistent with z = 3.5 where field galaxies demonstrate a clear peak in their redshift distribution.

A clear peak at z ≥ 3.5 in the redshift distribution

The redshift distribution of the galaxies with either a photometric or a spectroscopic redshift in the 6.9 Õ ◊10 Õ GOODS-ALMA field presents a peak at z ≥ 3.5 (Fig. 3.6). This peak is at a 3.5 ‡ significance among the 1373 galaxies at 3 AE z AE 4 ( Fig. 3.6). The redshift bin size is taken to be 0.03 to optimize the peak in the redshift distribution, which was previously identified in Straatman et al. (2016, Fig. 23) using the ZFOURGE catalog.

Here we increase by a factor of three the total number of galaxies with a spectroscopic redshift that fall within the redshift range supplement the already existing 38 spectroscopic redshifts originally listed in the ZFOURGE catalog and used by [START_REF] Straatman | The FourStar Galaxy Evolution Survey (ZFOURGE): Ultraviolet to Far-infrared Catalogs, Medium-bandwidth Photometric Redshifts with Improved Accuracy, Stellar Masses, and Confirmation of Quiescent Galaxies to z 3.5[END_REF].

The 121 galaxies with spectroscopic redshifts at 3 AE z spec AE 4 show a clear peak at 3.470 AE z spec AE 3.480 containing ten galaxies (Fig. 3.6). These ten galaxies have a mean redshift at z spec = 3.472. This is consistent with the peak at z ≥ 3.5 shown by the 1373 galaxies with either spectroscopic or photometric redshifts.

3.5.3 Optically-dark galaxies at z ≥ 3.5

We find that four out of the six optically-dark galaxies exhibit a redshift that is consistent with being located in the same redshift peak at z≥3.5, which corresponds to an over-density centered on the most massive of them, AGS24, as to be discussed below. We may even count five out of the six if we include AGS4, which is at z spec = 3.556 and at a distance of 3 Õ from AGS24. Accounting for the redshifts of the sources, this corresponds to 70.8 cMpc (comoving) and 15.5 pMpc (proper). The comoving distance is typical of those found in distant super-clusters while the proper distance is consistent with those of proto-clusters, which suggests that AGS4 could potentially be a member of the proto-cluster as well.

We start by describing in detail the properties of AGS24, because it is the most massive of all the optically-dark galaxies in GOODS-ALMA and even among all GOODS-South galaxies at z > 3 and constitutes an excellent candidate for a future BCG galaxy. The remaining three optically-dark galaxies at z≥3.5 are discussed in the next section.

F 2mm log L CO H F 3GHz F 6GHz L 1.4GHz [deg] [deg] log [M § ] log [L § ] [M § yr ≠1 ] [mJy] [mJy] log [L § ]
[mag]

[µJy]

[µJy]

[W Hz ≠1 ]

(1)

(2) (15) Flux densities at 6 GHz: the flux density of AGS4 is the peak flux derived using PyBDSF 1 , flux densities of AGS17, 24 and 25 are the peak flux densities in the 6GHz image, AGS11 and AGS15 don't have peak flux density higher than 3 ‡;

(3) (4) (5) (6) (7)
(17) Luminosity at 1.4GHz: derived from the flux density at 3GHz using Eq. 3.1.

3.5

An over-density at z ≥ 3.5 in GOODS-ALMA AGS24 has been detected with a signal-to-noise-ratio of 4.9 ‡ and 8.7 ‡ in the GOODS-ALMA 1.1mm image and in the 2 mm continuum image of the ALMA spectroscopic scan follow-up (red and green contours in Fig. 3.7). This galaxy exhibits a lower S/N ratio in the GOODS-ALMA image tapered at 0 ÕÕ .6 (3.9 ‡) than in the image at the original resolution of 0 ÕÕ .29 (4.9 ‡), which suggests that the dust emission is very compact. This optically-dark galaxy exhibits a clear radio counterpart at 6 GHz (3.7 ‡) and 3 GHz (5.7 ‡). Despite being absent from the list of IRAC sources in the field given in the S-CANDELS catalog [START_REF] Ashby | S-CANDELS: The Spitzer-Cosmic Assembly Near-Infrared Deep Extragalactic Survey. Survey Design, Photometry, and Deep IRAC Source Counts[END_REF], this source does exhibit clear IRAC emission at 3.6 and 4.5 µm that is partly blended with the three surrounding very nearby galaxies that are clearly detected in the H and K s bands. The emission of AGS24 in the H and K s bands is marginal but was estimated after de-blending the contribution from the three neighboring sources and used to determine its photometric redshift.

After de-blending 3 , we derive a photometric redshift of z AGS24 = 3.58 +0.49 ≠0.38 . Assuming this redshift, the excess emission in the V band may correspond to the Ly-emission line (Fig. 3.3-Bottom-left). However, we analyzed the VLT/MUSE data obtained in this region but did not find any evidence for Ly-emission despite the high sensitivity of MUSE (rms = 4◊10 20 erg s ≠1 cm ≠2 A ≠1 [START_REF] Herenz | The MUSE-Wide survey: A first catalogue of 831 emission line galaxies[END_REF]. We conclude that the excess V band emission remains uncertain and may be due to noise fluctuation.

The three neighboring galaxies are located at similar distances of ≥1 ÕÕ .5 (equivalent to 11.3 kpc at z ≥ 3.5) from AGS24 (Fig. 3.7 and Table 3.2). They all have a redshift of z≥3.5, including two spectroscopic redshifts -ID ZFOURGE = 10627, z spec = 3.478 and ID CDFS = 007493, z spec = 3.482 (VANDELS DR3) -and one photometric redshift -ID ZFOURGE = 10672, z phot = 3.53 [START_REF] Straatman | The FourStar Galaxy Evolution Survey (ZFOURGE): Ultraviolet to Far-infrared Catalogs, Medium-bandwidth Photometric Redshifts with Improved Accuracy, Stellar Masses, and Confirmation of Quiescent Galaxies to z 3.5[END_REF]. As a 44 Chapter 3 GOODS-ALMA: optically-dark ALMA galaxies shed light on a cluster in formation at z = 3.5 result, the group of four galaxies appears to be physically connected and makes a dense and compact association of galaxies typical of what can be found at the center of distant proto-clusters [START_REF] Springel | Simulations of the formation, evolution and clustering of galaxies and quasars[END_REF][START_REF] De Lucia | The hierarchical formation of the brightest cluster galaxies[END_REF].

Altogether, the following reasons suggest that AGS24 is located at the redshift of z≥3.472 (Section 3.5.2) of the proto-cluster: (i) the three neighbors of AGS24 have z≥3.472, (ii) the photometric redshift of AGS24 is consistent with this redshift, (iii) AGS24 belongs to the highly improbable association of optically-dark galaxies that fall in the region of the z≥3.472 proto-cluster.

Assuming this redshift, we estimate the total infrared luminosity of AGS24 to be L IR = 10 12.31±0.11 L § fitting the rest-frame 195, 245, 490 µm emission of the galaxy that corresponds to the observed 870 µm, 1.13 and 2 mm using the typical far-infrared SED of a MS galaxy at this redshift [START_REF] Schreiber | The Herschel view of the dominant mode of galaxy growth from z = 4 to the present day[END_REF]. We also checked that we obtained a consistent total infrared luminosity after fitting these measurements with a Draine et al. ( 2007) SED, which gives the same result within 10%.

Assuming z = 3.472, the stellar mass of AGS24 is M ı = 10 11.32 +0.02 ≠0.19 M § (adopting the [START_REF] Calzetti | The Dust Content and Opacity of Actively Star-forming Galaxies[END_REF] attenuation law and a delayed exponentially declining SFH). It is ≥ 10 times more massive than the galaxy presented in [START_REF] Ginolfi | Molecular gas on large circumgalactic scales at z = 3.47[END_REF] which used to be the most massive galaxy at z = 3≥ 4 in this field. We note that the stellar mass was derived by fitting the near UV to near-infrared photometry, similar to the method we used for our galaxies. In order to test the robustness of the value of the stellar mass of AGS24, we recompute it assuming various star formation histories (SFH) to see whether this mass could fall down by a large amount. We use the SED fitting code FAST++ 4 to test a delayed exponentially declining and an exponentially declining SFH as well as a truncated SFH, with the possibility to add a second burst. We also test the impact of varying the attenuation law using the code CIGALE [START_REF] Boquien | CIGALE: a python Code Investigating GALaxy Emission[END_REF] with the attenuation laws from [START_REF] Calzetti | The Dust Content and Opacity of Actively Star-forming Galaxies[END_REF], [START_REF] Fall | A Simple Model for the Absorption of Starlight by Dust in Galaxies[END_REF], and the modified Charlot and [START_REF] Fall | A Simple Model for the Absorption of Starlight by Dust in Galaxies[END_REF] law in MAGPHYS [START_REF] Da Cunha | A simple model to interpret the ultraviolet, optical and infrared emission from galaxies[END_REF]. The results are listed in Table 3.3. They show that despite these wide variety of SFH and attenuation laws, the stellar mass of AGS24 remains within 10 11.31 and 10 11.52 M § .

With a redshift of z ≥ 3.5, AGS24 is the most massive galaxy in the GOODS-ALMA field above z = 3. The other similarly massive candidate galaxies in this redshift range either have a strong AGN potentially contaminating the estimate of the stellar mass or highly unreliable redshifts. Two galaxies without an AGN at z > 3 have similarly large stellar masses in the GOODS-ALMA field (ID ZFOURGE = 11505 and 16410, marginally detected in the K s -band with S/N = 5.0, 5.5, at z phot = 5.36, 4.73). They both have very high ‰ 2 values of 126.6 and 43.0 associated with their photometric redshifts. Only 3% and 19% of the ZFOURGE galaxies have photometric redshifts with such high ‰ 2 values. Therefore, we consider the redshifts of these two galaxies not reliable.

We inspect the possible contamination of an AGN on the near-and mid-infrared emission which may result in an overestimation of the stellar mass. We calculate the infrared to radio flux ratio, q TIR = 2.05±0.18 (see Eq. 3.2), where the 1.4GHz luminosity, L 1.4 GHz = 1.87◊10 24 W Hz ≠1 , is converted from the JVLA 3GHz emission (5.7 ‡) of AGS24 assuming a radio spectral index of -= ≠0.8, as we did for AGS4. Within the rms, the infrared to radio flux ratio agrees with the value q TIR = 2.16 ± 0.06 for star-forming galaxies at redshift z ≥ 3.5 by [START_REF] Delhaize | The VLA-COSMOS 3 GHz Large Project: The infraredradio correlation of star-forming galaxies and AGN to z 6[END_REF], indicating no radio excess of this galaxy. We also checked that AGS24 does not have a counterpart in the deepest Chandra 7Ms source catalog [START_REF] Luo | The Chandra Deep Field-South Survey: 7 Ms Source Catalogs[END_REF]. We searched for extended diffuse emission in the Chandra 7Ms image but found no robust evidence for such emission that could be associated with an intra-cluster medium. Therefore, we conclude that AGS24 is the most massive galaxy at z > 3 without an AGN in the GOODS-ALMA field. [START_REF] Forrest | An Extremely Massive Quiescent Galaxy at z = 3.493: Evidence of Insufficiently Rapid Quenching Mechanisms in Theoretical Models[END_REF] recently spectroscopically confirmed (z spec = 3.493) a similarly extremely massive quiescent galaxy in the VIDEO XMM-Newton field (Annunziatella et al., in prep.) that is spectroscopically confirmed (z spec = 3.493) and interpreted as experiencing a rapid downfall of star formation.

The predicted number of galaxies that are as massive or more massive than AGS24 in the GOODS-ALMA field at 3 < z < 4.5 is 1.15, based on the galaxy stellar mass function (SMF) derived from the observations in the COSMOS field [START_REF] Davidzon | The COSMOS2015 galaxy stellar mass function . Thirteen billion years of stellar mass assembly in ten snapshots[END_REF]. [START_REF] Pillepich | First results from the IllustrisTNG simulations: the stellar mass content of groups and clusters of galaxies[END_REF] compared the SMF of [START_REF] Davidzon | The COSMOS2015 galaxy stellar mass function . Thirteen billion years of stellar mass assembly in ten snapshots[END_REF] to the one from the IllustrisTNG simulation and found them consistent. Thus the existence of one massive galaxy, AGS24, is consistent with the existing observation and simulation.

3.5.3.2 Physical properties of the optically-dark galaxies AGS11, AGS15 and AGS17 at z≥3.5

• AGS11 is detected at 1.1mm with a S/N = 5.71 in F18 (also see red solid contours in Fig. 3.8-Top) and 2 mm with a S/N = 16.8 (green dashed contours in Fig. 3.8-Top). A neighboring galaxy is identified in ZFOURGE that matches the K s and IRAC emission (magenta "x" in Fig. 3.8-Top). This object is located at 0 ÕÕ .45 from the ALMA detection. It has been attributed with a photometric redshift of z 7589 phot = 4.82 in the ZFOURGE catalog. However, this redshift is highly uncertain with flag use = 0, hence highly unreliable. Data at hand do not allow us to determine whether this object is the counterpart of the ALMA detection, AGS11, or like several other optically dark sources studied here, an independent galaxy seen in projection. The optical emission at the ALMA detection position is within the noise fluctuation. Therefore, we do not attributed the ZFOURGE ID and the associated redshift to AGS11. Instead, we consider the association of the optically dark galaxies with the over-density at z ≥ 3.5 as potential evidence that AGS11 has a redshift matching this over-density. In Table 3.1 we decided to attribute AGS11 with the z peak = 3.472 3.5 An over-density at z ≥ 3.5 in GOODS-ALMA of the over-density. Assuming z AGS11 = 3.472, the infrared luminosity we derived for AGS11 is L IR = 10 12.94±0.07 L § .

• AGS15 is located at 0 ÕÕ .59 to a optically bright galaxy (ID CANDELS = 3818, cyan "x" in Fig. 3.9). It is detected at 5.22 ‡ in the 1 mm image (F18, see also red contours in Fig. 3.9). This detection is further confirmed in the 2 mm continuum at 15.3 ‡ in the follow up ALMA spectroscopic scan (green dashed contours in Fig. 3.9). In the K s -band image, the infrared emission from AGS15 and from the optically bright galaxy is blended, and the two galaxies are confused as one (ID ZFOURGE = 6755, magenta "x" in Fig. 3.9) in the ZFOURGE catalog. The redshift derived by the ZFOURGE team is z 6755 phot = 3.46 and the stellar mass is M ı = 10 9.86 M § . We notice that there is a high uncertainty in the SED fitting by the ZFOURGE team, with ‰ 2 = 45.4 which is higher than 84% of all the galaxies in the ZFOURGE catalog. If it is the counterpart of AGS15, then the low stellar mass will make AGS15 an extreme case, i.e., 10 times less massive compared to the other ALMA detections. After de-blending AGS15 and the optically bright galaxy, as shown in Fig. 3.9 and Fig. 3.3-Top-right, we find that in fact AGS15 has an H band magnitude higher than the detection limit, H = 27.11 AB. We fit the SED with EAzY [START_REF] Brammer | EAZY: A Fast, Public Photometric Redshift Code[END_REF] and FAST++ 4 to derive the photometric redshift and the stellar mass. The z peak derived for AGS15 is at z AGS15 phot = 3.96 +0.80 ≠0.78 . Hence the galaxy has, within the uncertainties on the photometry, a photometric redshift that encompasses the redshift peak at z peak = 3.472. Assuming this redshift, we obtain the infrared luminosity of AGS15 to be L IR = 10 12.78±0.08 L § , the SFR to be SFR = 1042 +181 ≠183 M § yr ≠1 and the stellar mass to be M ı = 10 10.56 +0.01 ≠0.41 M § .

• AGS17 is detected at 1.1 mm with a S/N = 5.01 (F18, also see red solid contours in Fig. 3.8-Bottom) and with a S/N = 15.9 in the 2 mm continuum (green dashed contours in Fig. 3.8-Bottom).

It is located between two bright optical emitters, ID CANDELS = 4414, (z 4414 phot = 0.03, Pannella et al., 2015) at 0 ÕÕ .27, and ID CANDELS = 4436, (z 4436 phot = 0.95, fitted with EAzY) at 0 ÕÕ .57 (cyan "x"s in Fig. 3.8-Bottom). These two emitters are also clearly seen in the K s -band image, however, they are not resolved in the ZFOURGE catalog (magenta "x" in Fig. 3.8-Bottom, ID ZFOURGE = 6964). The redshift derived from this blended emission is z 6964 phot = 1.85 which is not consistent with either the redshifts of the optical emitters.

Assuming that the emission line detected in our ALMA spectroscopic follow-up is the CO transition as discussed in Section 3.3.2, AGS17 has a spectroscopic redshift z AGS17 spec = 3.467, which puts it at a distance of 4.30 cMpc from AGS24, assuming z AGS24 = z peak = 3.472. Hence it falls inside the proto-cluster as the distance is shorter than the typical size of a proto-cluster (≥14.5 cMpc) at z ≥ 3.5 in simulations [START_REF] Muldrew | What are protoclusters? -Defining high-redshift galaxy clusters and protoclusters[END_REF]. The low probability to have a galaxy fall in projection with the structure that is located at the redshift corresponding to the CO transition makes us favor this transition. Then the stellar mass is M ı = 10 10.59 +0.29 ≠0.19 M § based on the IRAC band emission and the infrared luminosity derived from the SED fitting using Herschel and ALMA data is L IR = 10 13.08±0.02 L § .

3.5.4 Spatial distribution of galaxies at z ≥ 3.5 in the GOODS-ALMA field

Local over-density (") of individual galaxies

We calculate the over-density (" = fl / f) of the 364 galaxies at 3.42 AE z AE 3.57 in GOODS-ALMA. This redshift range covers a distance of 125 cMpc which is consistent with the size of a large scale structure, a (proto-)supercluster of ≥ 60 ◊ 60 ◊ 150 cMpc at z ≥ 2.45, identified by [START_REF] Cucciati | The progeny of a cosmic titan: a massive multicomponent proto-supercluster in formation at z = 2.45 in VUDS[END_REF]. We derive a dispersion of ‡(") = 0.20 among the 364 galaxies. We do that by comparing the projected local galaxy density within 2 cMpc (fl) of a galaxy, to the average number density over the GOODS-ALMA field (f = 1.32 cMpc ≠2 or 5.13 arcmin ≠2 ).

Seven of the detections in the GOODS-ALMA field reside in this narrow redshift range, AGS1, 4, 5, 11, 15, 17, and 24, five (underlined) out of the seven are optically-dark. The z AGS1 spec = 3.442 of AGS1 comes from VANDELS DR3, zflag = 1, i.e., 50% probability to be correct, while we note that it is recorded as z AGS1 spec = 2.309 in F18. AGS5 has a photometric redshift of z AGS5 phot = 3.46 [START_REF] Straatman | The FourStar Galaxy Evolution Survey (ZFOURGE): Ultraviolet to Far-infrared Catalogs, Medium-bandwidth Photometric Redshifts with Improved Accuracy, Stellar Masses, and Confirmation of Quiescent Galaxies to z 3.5[END_REF]. As mentioned in Section 3.4.1, AGS4 has a redshift of z AGS4 spec = 3.556.

At the same time, eight massive galaxies fall in this redshift range in the ZFOURGE catalog. They have a stellar mass, log [M ı /M § ] > 10.5, which is equivalently massive as the ALMA detections (F18). They have a median stellar mass of M ı = 10 10.82 M § . These eight massive galaxies, as well as the seven ALMA detections all show a local over-density (" > 1). Out of the 15 massive galaxies, AGS24 shows the most prominent clustering, " AGS24 = 2.06, hence a > 5 ‡ excess with respect to the average galaxy surface density. Hence AGS24 is both the most massvie galaxy at z > 3 in the GOODS-ALMA field with no AGN and it is located in the densest surface density peak at z ≥ 3.5. This makes it an excellent candidate for a future BCG as we discuss in Section 3.5.5. The degree of over-density of the four other HST-dark galaxies are " AGS11 = 1.46 (> 2 ‡ excess), " AGS15 = 1.64 (> 3 ‡ excess), " AGS17 = 1.76 (≥ 4 ‡ excess), " AGS4 = 1.39 (≥ 2 ‡ excess), and for the other two ALMA detection with optical counterparts, it is " AGS1 = 1.21, " AGS5 = 1.03.

Regardless of mass, when focusing on the galaxies in the densest regions, i.e., " > 5 ‡("), we found in total 12 galaxies in the ZFOURGE catalog. It turns out that they all lie within 50 ÕÕ of AGS24 (red circle in Fig. 3.11) and the barycenter of these galaxies falls exactly at AGS24. The most massive of the 12 galaxies is ID ZFOURGE = 10672, M ı = 10 10.76 M § . It is at 1 ÕÕ .5 from AGS24.

In Fig. 3.10, we show the over-density as a function of stellar mass of the 364 galaxies at 3.42 < z < 3.57 in the GOODS-ALMA field and the ALMA detected galaxies. We can see that as the stellar mass increases, the galaxies tend to fall in denser regions, which is in line with what [START_REF] Muldrew | What are protoclusters? -Defining high-redshift galaxy clusters and protoclusters[END_REF] found in Millennium Simulation [START_REF] Springel | Simulations of the formation, evolution and clustering of galaxies and quasars[END_REF] on galaxies at z = 2. They conclude that the most massive galaxies at high redshift reside in proto-clusters given the little difference in the environments of only proto-cluster galaxies and those of all the massive galaxies. Besides, the ALMA detections are the most massive ones in the field and reside in the densest regions. AGS24, as the most massive one in the field, resides in the densest region in the field. Therefore, these optically dark galaxies revealed by ALMA make up for what has been missed from optical observations. The relation between their dusty and massive nature and the dense environment they reside in provides evidence of environmental effects on galaxy evolution.

Projected density map over the GOODS-ALMA field

We construct the projected galaxy density in Fig. 3.11. Similar to the method used in Forrest et al.

(2017), we create a grid with a cell size of 0 ÕÕ .9 ◊ 0 ÕÕ .9 in the GOODS-ALMA field and calculate the over-density (" = fl / f) for each cell. 364 galaxies (40 with z spec ) at 3.42 AE z AE 3.57 are used to produce the map. The galaxies with spectroscopic redshifts are marked as white dots in Fig. 3.11. An overdense region comes out in the South-East region as indicated by the zoom-in box in Fig. 3.11. The 4 ‡ peak of this over-density happens to be very close to AGS24, the galaxy with the strongest over-density (" AGS24 = 2.06) this is consistent with the fact that it is in a highly clustered environment. Out of the 364/40 (redshift/z spec ) galaxies in the 69 arcmin 2 field, 126/18 reside in this 15 arcmin 2 box, and 26/3 galaxies in this redshift slice fall within 50 ÕÕ (2 arcmin 2 ) of AGS24.

We notice that the over-density shown in the projected density map encompasses four out of the six HST-dark galaxies detected by ALMA (AGS 11,15,17,24,stars in Fig. 3.11). The overlap of an over-density of ALMA detected optically-dark galaxies and an over-density of optically detected galaxies shows that these ALMA detected galaxies are good tracers of structures in the early Universe.

This over-density has been classified as a proto-cluster in previous publications. [START_REF] Franck | The Candidate Cluster and Protocluster Catalog (CCPC) of Spectroscopically Identified Structures Spanning 2.74 &lt[END_REF] identified it in a list of proto-clusters at 2.74 < z < 3.71 built out of ≥ 14000 spectroscopic redshifts (with 604 of them finally used). They defined proto-clusters as galaxy densities greater than the field density by a factor 7 within a radius of 2 Õ and a redshift range of "z < 0.03. One of their proto-clusters, CCPC-z34-002, has a redshift of z = 3.476, hence consistent with the redshift peak of z = 3.472 studied here. We note that the CCPC-z34-002 density peak, roughly estimated from the distribution of only 23 galaxies within a radius of 20 cMpc, does not correspond to the location of AGS24 but falls only 10 ÕÕ ) including 26(3) galaxies. The yellow stars refer to the HST-galaxies AGS 11, 15 and 17 located also in this larger structure. The green circles represent the AGS 1 and 5 which are also in this redshift range but not optically-dark. The white circles show the 40 galaxies with spectroscopic redshifts. We note that the two white circles falling on the red star of AGS24 are two galaxies dissociated from AGS24. The magenta square denotes the center of the proto-cluster CCPC-z34-002 identified by [START_REF] Franck | The Candidate Cluster and Protocluster Catalog (CCPC) of Spectroscopically Identified Structures Spanning 2.74 &lt[END_REF]. The cyan square denotes Candels5001, which used to be the most massive galaxy at 3 < z < 4 in this field [START_REF] Ginolfi | Molecular gas on large circumgalactic scales at z = 3.47[END_REF]. The 50 ÕÕ scale in the figure is equivalent to a proper distance of 0.37 Mpc at z = 3.5.

3.5

An over-density at z ≥ 3.5 in GOODS-ALMA from AGS17, and the typical diameter of the proto-clusters studied by [START_REF] Franck | The Candidate Cluster and Protocluster Catalog (CCPC) of Spectroscopically Identified Structures Spanning 2.74 &lt[END_REF] of 4 arcmin matches the zoom shown in Fig. 3.11-Right. In the following, we will consider as a members of a candidate proto-cluster the galaxies of the redshift peak that fall in this area. This structure encompasses four out of six, hence 67 %, of the optically-dark galaxies identified in the GOODS-ALMA field.

This over-density was also identified by [START_REF] Forrest | Discovery of Extreme [O III]+H-Emitting Galaxies Tracing an Overdensity at z 3.5 in CDF-South[END_REF] who used a population of extreme emission line galaxies, with low stellar masses and by the VANDELS survey with a three-dimensional algorithm [START_REF] Guaita | The VANDELS survey: Discovery of massive overdensities of galaxies at[END_REF]. Moreover, only nine out of 131 the Ly--emitting galaxies selected from the VANDELS survey reside in their 13 detected over-densities at z > 2, indicating that Ly--emitting galaxies are not ideal tracers of over-densities. Finally, we note that [START_REF] Ginolfi | Molecular gas on large circumgalactic scales at z = 3.47[END_REF] observed the molecular gas content of the galaxy CANDELS-5001 (blue square in Fig. 3.11, M ı = 10 10.27 +0.38 ≠0.11 M § , z spec = 3.473) and found that this galaxy was surrounded by an extended emission of molecular gas spanning 40 kpc traced by the CO(4-3) transition. Furthermore they detect nine additional CO systems within a radius of 250 kpc from the massive galaxy and mostly distributed in the same direction as the CO elongated structure found in the central 40 kpc which they interpret as evidence for large-scale gas accretion on the galaxy. This large-scale accretion might be enhanced by the larger scale dark matter halo of the structure (see e.g. [START_REF] Rosdahl | Extended Ly-emission from cold accretion streams[END_REF]).

We calculate the distance between AGS1, 4, 5 and AGS24 to be 40.5 cMpc, 70.8 cMpc and 25.5 cMpc respectively. Despite their larger distance, AGS1 and 5 might be involved in a wider structure than the proto-cluster that includes AGS11, 15, 17, 24 and potentially also AGS4. This wider structure may be compared to the superclusters at high redshift discussed in [START_REF] Cucciati | The progeny of a cosmic titan: a massive multicomponent proto-supercluster in formation at z = 2.45 in VUDS[END_REF] and Toshikawa et al.

(2020).

Fig. 3.12: Comparison of the auto-correlation function (ACF) of the 364 galaxies at 3.42 AE z AE 3.57 in the ZFOURGE catalog (red dots), the cross-correlation function (CCF) between these galaxies and the ALMA detections (black triangles), and the cross-correlation function between these galaxies and the galaxies which are equivalently massive as the ALMA detections (green triangles), i.e., Mı > 10 10.5 M §.

52

Chapter 3 GOODS-ALMA: optically-dark ALMA galaxies shed light on a cluster in formation at z = 3.5

Two point correlation function (2PCF)

We calculated the auto-correlation function of the 364 galaxies (40 with z spec ) at 3.42 AE z AE 3.57 using a uniformly generated random sample in the GOODS-ALMA field, independent of mass (red dots in Fig. 3.12), the cross-correlation function between these 364 galaxies and the ALMA detections (black triangles in Fig. 3.12), and the cross-correlation function between these 364 galaxies and the massive galaxies which are equivalently massive as the ALMA detections, i.e., M ı > 10 10.5 M § out of the 364, (green triangles in Fig. 3.12). A Landy and Szalay (1993) estimator is adopted. Error bars are estimated from 1000 bootstrap samples. Despite of the large error bars, the two cross-correlations both show systematically higher excess possibility compared to the auto-correlation of the 364 galaxies in this redshift range. This indicates a clustering feature of the ALMA detected optically dark galaxies, as well as the equivalently massive galaxies.

3.5.5 Dynamical state of the proto-cluster at z ≥ 3.5

As discussed in Section 3.5.3.1, AGS24 is the most massive galaxy in the field at z > 3 (M ı = 10 11.32 +0.02 ≠0.19 M § ) and in particular at z ≥ 3.5, making it a good candidate for the future BCG of the over-density of galaxies if it becomes a galaxy cluster. The fact that it is surrounded by three galaxies with the same redshift of z ≥ 3.5 within 1 ÕÕ .5 from AGS24 (11.3 kpc at z ≥ 3.5 ) strengthens this hypothesis (Fig. 3.7 and Table 3.2).

To estimate the dark matter mass of this structure, we first calculated the total stellar mass of all the galaxies within 50 ÕÕ of AGS24 and then determined an upper limit of M h =10 15.0 M § to the dark matter mass based on the relation in [START_REF] Behroozi | The Average Star Formation Histories of Galaxies in Dark Matter Halos from z = 0-8[END_REF], assuming that this structure is already virialized. We also determined a lower limit of M h =10 13.3 M § by summing up the individual halo masses of the each galaxy within 50 ÕÕ of AGS24.

We note that the projected number density profile of the galaxies in the structure do not follow the slope expected for the NFW profile [START_REF] Navarro | The Structure of Cold Dark Matter Halos[END_REF] of a gravitationally bound cluster, but instead exhibit a shallow slope (Fig. 3.13). We show for comparison the projected number density profile of the most distant galaxy cluster to our knowledge, CL J1001+0220 at z = 2.506 (Wang et al., 2016a).

However, the structure shown in the density map (Fig. 3.11), as well as the fact that AGS24 is the most massive galaxy which falls at the density peak and has a reduced SFR compared to MS galaxies, suggests that this structure is in the process of viralization and that such an environment is influencing the evolution of AGS24, diminishing its star formation activity. Galaxies like AGS24 have been missed by previous studies given their dusty nature, despite being the most massive candidate galaxies that can be used as a beacon to trace the barycenter of the proto-clusters in the process of virialization.

3.5

An over-density at z ≥ 3.5 in GOODS-ALMA Fig. 3.13: Projected numbers of galaxies in the over-density as a function of their distance to AGS24 (black circles) and the black dashed line is the linear fitting to the profile. The red line shows the best-fit projected NFW profile [START_REF] Navarro | The Structure of Cold Dark Matter Halos[END_REF] of a galaxy cluster at z = 2.506, CL J1001+0220 (Wang et al., 2016a).

Conclusions

We have analyzed the properties of an ensemble of six optically dark galaxies, AGS4, 11, 15, 17, 24 and 25 identified in the GOODS-ALMA survey. They do not have optical counterparts in the deepest H-band based catalog down to H = 28.16 AB.

Out of the six sources initially classified as optically dark, a careful analysis revealed that two were associated with an H-band counterpart that was not identified in the H-band catalog missed due to blending. AGS4 and AGS15, that were identified as optically dark in F18, are extremely close to a bright optical source at 0 ÕÕ .50 and 0 ÕÕ .27 respectively, and as such they were wrongly considered as parts of the bright neighbor. However we showed that their SED can be used to perform a spectral de-confusion of the sources at very different redshifts. We note that the H-band magnitude of these galaxies is extremely faint since it is 25.2 AB and 27.1 AB for AGS4 and AGS15 respectively. The second one is below the detection limit of most H-band images. The definition of HST-dark or optically-dark is obviously a function of the depth of the HST images. All of six sources except AGS24 do exhibit a counterpart in the K s -band image but they were mistakenly associated with a bright neighbor, hence none was listed in the ZFOURGE catalog.

We performed a spectroscopic scan follow-up of five of them with ALMA (AGS4, 11, 15, 17 and 24). All five exhibit a clear 2 mm counterpart in the continuum with S/N = 14.7, 16.8, 15.3, 15.9 and 8.7 respectively, which reinforces their robustness. We detected one emission line for AGS4 (‹ obs = 151.44 GHz with a S/N = 8.58) and for AGS17 (‹ obs = 154.78 GHz with a S/N = 10.23). Taking into consideration the PDF of the photometric redshifts derived from the optical to near-infrared SEDs and the far-infrared SEDs as well as the infrared luminosities, we conclude that the spectroscopic redshifts of the two galaxies match the CO Close to 70 % (4/6) of the optically-dark galaxies reside in an over-density of galaxies at z ≥ 3.5 (AGS11, 15, 17 and 24, Fig. 3.11). In [START_REF] Wang | A dominant population of optically invisible massive galaxies in the early Universe[END_REF], the HST-dark galaxies exhibit a highly heterogeneous spatial distribution illustrated by their strong clustering measured by the cross-correlation functions. This is consistent with the present finding of a strong association of most of the blindly detected optically-dark galaxies in GOODS-ALMA of four out of six galaxies located in the same proto-cluster. In addition, the redshifts of these galaxies are all consistent with this over-density (Table . 3.1), indicating that these galaxies are tracing a galaxy cluster in formation.

We also notice that AGS24 is not only in the barycenter of the structure at z ≥ 3.5 but also the most massive galaxy in this structure (M ı = 10 11.32 +0.02 ≠0.19 M § ), suggesting that AGS24 is a candidate BCG in formation. In fact, AGS24 is the most massive galaxy without an AGN at z > 3 in the GOODS-ALMA field. Compared to the other five optically dark galaxies with high starburstness, AGS24 is the only main sequence galaxy (Table 3.1) indicating that it is influenced by the environment it resides in. The fact that AGS24 is a is a candidate BCG also indicates that the proto-cluster is in the process of virialization.

Conclusions

Extremely weak CO emission in IZw 18

Local metal-poor galaxies are ideal analogs of primordial galaxies with interstellar medium (ISM) barely enriched with metals. However, it is unclear whether carbon monoxide remains to be a good tracer and coolant of molecular gas at low metallicity.

Based on the observation with the upgraded NOEMA Interferometer, I report in this chapter an upper limit of CO J=2-1 emission in IZw18, pushing down the detection limit to be L Õ CO(2≠1) < 7.64 ◊ 10 3 K km s ≠1 pc ≠2 , ten times lower than previous studies. As one of the most metal poor galaxy, IZw18 shows extremely low CO content despite its vigorous star formation activities. Such low CO content relative to its infrared luminosity, star formation rate (SFR) and [C II] luminosity, compared to other galaxies, indicates a drastic change in the structure of the ISM at around a few percentages of Solar metallicity. Especially, the high [C II] luminosity relative to CO implies a larger molecular reservoir than the CO emitter in IZw18. I also obtain an upper limit of the 1.3 mm continuum overlapping the H I emission peak. This upper limit excludes a submilimeter excess in IZw18.

Introduction

Star formation takes place in molecular gas dominated by H 2 , except perhaps for that in the early universe. However, H 2 can only be excited at a temperature above 100 K, hence it cannot be observed directly in the cold molecular gas that fuels star formation. The second most abundant molecule, CO, has been demonstrated to be a powerful tracer of the molecular clouds in galaxies, but this application becomes complicated in metal-poor environments (see [START_REF] Bolatto | The CO-to-H2 Conversion Factor[END_REF], for a review).

In the early Universe, first galaxies form in the primordial gas with little elements heavier than hydrogen. Observationally, at z & 5, normal star-forming galaxies with star formation rates (SFRs) of ten to a few hundred solar masses per year indeed have a low dust content (Walter et al., 2012a;[START_REF] Capak | Galaxies at redshifts 5 to 6 with systematically low dust content and high [C II] emission[END_REF], comparable to local metal-poor galaxies. Nevertheless, the predicted CO flux in these high-z galaxies is in the order of µJy and remains challenging with the state-of-the-art submilimeter array, ALMA. Detailed study on the metal-poor interstellar medium (ISM) relies on their local analogs.

Despite extensive search for CO emission in local dwarf galaxies [START_REF] Leroy | The Low CO Content of the Extremely Metal-poor Galaxy I Zw 18[END_REF][START_REF] Schruba | Low CO Luminosities in Dwarf Galaxies[END_REF][START_REF] Cormier | The molecular gas reservoir of 6 low-metallicity galaxies from the Herschel Dwarf Galaxy Survey. A ground-based follow-up survey of CO(1-0), CO(2-1), and CO(3-2)[END_REF]Hunt et al., 2015a;[START_REF] Warren | CARMA CO Observations of Three Extremely Metal-poor, Star-forming Galaxies[END_REF]Shi et al., 2015a), the detection rate decreases sharply in galaxies at metallicity lower than 1/5 Solar metallicity (Z § )1 . The extremely faint CO emission poses questions on the existence of molecular gas in such galaxies. On the other hand, CO may not be an ideal tracer of molecular gas [START_REF] Grenier | Unveiling Extensive Clouds of Dark Gas in the Solar Neighborhood[END_REF][START_REF] Wolfire | The Dark Molecular Gas[END_REF][START_REF] Shi | Carbon monoxide in an extremely metal-poor galaxy[END_REF] in low-metallicity environments. Because of the low dust content, the radiation field in metal-poor galaxies are enhanced. In this case, H 2 molecules self-shield while CO can be disscociated easily. Low CO content is then expected in metal-poor galaxies. On the other hand, exposed to the hard radiation field, CO molecules can be easily photodissociated and produce ionized carbon, [CII].

[CII] could come from ionized gas as well as neutral gas and the surface of photodissociation regions (PDRs). However, the fractions of [CII] emission from ionized gas derived from observation constraints are systematically lower than the one from simulations [START_REF] Accurso | Deriving a multivariate -CO conversion function using the [C II]/CO (1-0) ratio and its application to molecular gas scaling relations[END_REF][START_REF] Cormier | The Herschel Dwarf Galaxy Survey. II. Physical conditions, origin of [C II] emission, and porosity of the multiphase low-metallicity ISM[END_REF]. Hence it's still unclear if it is feasible to use [CII] to trace H 2 gas. So far, among the galaxies below 10% Z § , only Sextans B (7% Z § ), has a robust CO detection [START_REF] Shi | Carbon monoxide in an extremely metal-poor galaxy[END_REF][START_REF] Shi | Oversized Gas Clumps in an Extremely Metal-poor Molecular Cloud Revealed by ALMA's Parsec-scale Maps[END_REF]. The impact of low metal abundance on the star formation requires further exploration of the most metal poor galaxies. In this work, we used the upgraded NOEMA Interferometer to observe the CO J=2-1 emission in IZw18, one of the galaxies with the lowest metallicity in the local universe (≥ 3% Z § , Izotov and Thuan, 1999a). IZw18 is a proto-type of blue compact dwarfs located at a distance of 18.2 Mpc (Aloisi et al., 2007a). The active star formation therein [START_REF] Hunt | The radio continuum of the extremely metal-poor blue compact dwarf galaxy I Zw 18[END_REF] indicates the presence of molecular gas. [START_REF] Leroy | The Low CO Content of the Extremely Metal-poor Galaxy I Zw 18[END_REF] obtained an upper limit of the CO(1-0) emission (L Õ CO < 10 5 K km s ≠1 pc 2 ) using the IRAM plateau de Bure interferometer, which however is not deep enough to conclude whether IZw 18 has a normal CO content relative to other physical properties. Here we put further constraints on the CO content in IZw18 by pushing down the detection limit of L Õ CO by 10 times using NOEMA Interferometer after its Phase II upgrade.

Observations

The observations of IZw18 were carried out with the NOEMA Interferometer on 2017 December 24 (obs-17) and 2019 March 23 (obs-18) for a total of 16 hrs (11.6 hrs on source) with configuration D. We processed the data at IRAM/Grenoble using the GILDAS package. The final CO J = 2-1 datacubes of obs17, obs18 and the merge of the two, have beam sizes of 1 ÕÕ 83 ◊ 1 ÕÕ 60 (obs-17), 1 ÕÕ 71 ◊ 1 ÕÕ 41 (obs-18) and 1 ÕÕ 71 ◊ 1 ÕÕ 48 (merged), respectively, a frequncy resolution of 0.2MHz (0.26 km s ≠1 at 230 GHz), and rms of 4.7 mJy/beam, 5.9 mJy/beam and 4.6 mJy/beam, respectively. IZw18 was entirely covered by the 22 ÕÕ (FWHM) primary beam.

Results

CO J=2-1

We smoothed the data to a velocity resolution of 3 km s ≠1 , consistent with the CO line width found in the most metal poor galaxies in the literature [START_REF] Shi | Carbon monoxide in an extremely metal-poor galaxy[END_REF]. We found one marginal detection (-= 9 h 34 m 02 s 00, " = 55°14 Õ 28 ÕÕ .81) at the peak of the stellar emission, as indicated by the red contour superimpose on the HST V -band image (Fig. 4.1-Left). It covers an area defined by the 3- ‡ contour of ≥1 ÕÕ .52 ◊ 1 ÕÕ .52, approximately the size of the synthesized beam. We plot the spectra from the merged, obs-17 and obs-18 data cubes in Fig. 4.1-Right. A spectral signal at 3- ‡ is seen at 46 km s ≠1 , as a tentative detection. The peak fluxes derived from the peak channels have signal-to-noise ratios of 3.47, 2.73 and 3.45 for the merged, obs-17 and obs-18 data cubes. Hereafter we use the result from the merged data cube, and treat this marginal signal as an upper limit. The 3- ‡ upper limit of the CO J=2-1 flux is S CO(2≠1) = 2.48 mJy and the corresponding CO luminosity is then derived to be L Õ CO(2≠1) = 7.64 ◊ 10 3 K km s ≠1 pc ≠2 using the formulation of [START_REF] Solomon | Molecular Gas at High Redshift[END_REF]. The CO emission tends to reside in small clumps as what was found in another extremely metal poor galaxy, Sextans A [START_REF] Shi | Oversized Gas Clumps in an Extremely Metal-poor Molecular Cloud Revealed by ALMA's Parsec-scale Maps[END_REF]. We expect the missing diffuse emission should not contribute much to the upper limit, since most CO clumps are very compact at low metallicity conditions [START_REF] Glover | Modelling CO formation in the turbulent interstellar medium[END_REF][START_REF] Glover | On the relationship between molecular hydrogen and carbon monoxide abundances in molecular clouds[END_REF].

1.3 mm continuum

We created a continuum map after removing channels with covering emission. The continuum map is smoothed to have an angular resolution of 2 ÕÕ 21 ◊ 2 ÕÕ 06 with an rms noise of 24 µJy. The peak flux of 77 µJy falls at -= 9 h 34 m 02 s 00, " = 55°14 Õ 26 ÕÕ 30 which is only ≥1 ÕÕ from the flux peak of the HI emission of IZw18 [START_REF] Lelli | Dynamics of starbursting dwarf galaxies: I Zw 18[END_REF]. This is the only 3 ‡ signal within the primary beam.

Discussion

In this section, we discuss how the upper limits of the CO luminosity and the 1.3 mm continuum flux would constrain our current understanding on the star formation process in the extremely metal poor environment in IZw18.

Discussion

We compare IZw18 to the star-forming regions in four extremely metal poor galaxies (12 + log (O/H) < 8) from Shi et al. (2015a) and [START_REF] Shi | Carbon monoxide in an extremely metal-poor galaxy[END_REF], seven metal poor dwarf galaxies from the Herschel Dwarf Galaxy Survey (DGS Cormier et al., 2014, compiled in ), 16 nearby dwarf galaxies in [START_REF] Schruba | Low CO Luminosities in Dwarf Galaxies[END_REF] and nearby galaxies compiled therein, eight metal poor galaxies from Hunt et al. (2015a), local galaxies with stellar mass higher than 10 9 M § from the xCOLD GASS survey [START_REF] Saintonge | xCOLD GASS: The Complete IRAM 30 m Legacy Survey of Molecular Gas for Galaxy Evolution Studies[END_REF], and massive, infrared bright, local star-forming galaxies at Solar metallicity from [START_REF] Gao | The Star Formation Rate and Dense Molecular Gas in Galaxies[END_REF].

SED and Submilimetre excess

We first investigate the Spectral Energy Distribution (SED) of IZw18 taking into account the 3- ‡ upper limit at 1.3 mm. All other photometric data are taken from Hunt et al. (2015a). We performed the SED fitting with CIGALE [START_REF] Noll | Analysis of galaxy spectral energy distributions from far-UV to far-IR with CIGALE: studying a SINGS test sample[END_REF] adopting a delayed star formation history, a Draine et al. (2007) dust model, which is consistent with the method used for the star-forming regions in Shi et al. (2015a) and [START_REF] Shi | Carbon monoxide in an extremely metal-poor galaxy[END_REF], and a radio component. The infrared luminosity derived from the fit is 2.2 ◊ 10 7 L § , consistent with the one from Hunt et al. (2015a), the stellar mass is 4.33 ◊ 10 6 M § , within the uncertainty of the stellar masses derived from stellar mass-to-light ratios at K band and R band [START_REF] Fumagalli | Testing Models for Molecular Gas Formation in Galaxies: Hydrostatic Pressure or Gas and Dust Shielding?[END_REF][START_REF] Lelli | Dynamics of starbursting dwarf galaxies: I Zw 18[END_REF].

We also explore the existence of the excess at submilimeter wavelengths (e.g., [START_REF] Galliano | ISM properties in low-metallicity environments. II. The dust spectral energy distribution of NGC 1569[END_REF][START_REF] Lisenfeld | Sub)millimetre emission from NGC 1569: An abundance of very small grains[END_REF][START_REF] Galametz | Probing the dust properties of galaxies up to submillimetre wavelengths. II. Dust-to-gas mass ratio trends with metallicity and the submm excess in dwarf galaxies[END_REF]Rémy-Ruyer et al., 2013a) in IZw18. It does not have detections at the far-IR wavelengths longer than 160 µm. The 3 ‡ upper limit at 1.3 mm places a strong constraint on the (sub)milimeter end of the dust emission. We fitted the far-IR SED with the modified blackbody model and derived a lower limit of the dust emissivityto be 2.1, which suggests no submilimetre excess in IZw18. The infrared luminosity and SFR (Top-left) versus L Õ CO(1≠0) (Bottom-left) as well as the SFR/L Õ CO(1≠0) as a function of metallicity. The upper limit of IZw18 is show as a black circle with an arrow, in comparison with the galaxies described in Section 5.4. In the two panels on the left, the blue dashed lines show the linear correlation by [START_REF] Gao | The Star Formation Rate and Dense Molecular Gas in Galaxies[END_REF] and the red dashed lines show the linear fit of the metal poor galaxies (Shi et al., 2015a;[START_REF] Shi | Carbon monoxide in an extremely metal-poor galaxy[END_REF]Hunt et al., 2015a;Cormier et al., 2014, DGS). In the right panel, the blue solid line denote the best fit of the metal poor galaxies in Hunt et al. (2015a) where galaxies in [START_REF] Schruba | Low CO Luminosities in Dwarf Galaxies[END_REF] are also included. The black solid line shows a power law fitting all the galaxies in the plot.

In Fig. 4.3, we plot the total infrared luminosity (8 -1000 µm) versus L Õ CO(1≠0) and SFR versus L Õ CO(1≠0) . These two correlations have been well estabilished in massive star-forming galaxies (e.g., [START_REF] Gao | The Star Formation Rate and Dense Molecular Gas in Galaxies[END_REF] as CO molecules and L IR trace H 2 molecular clouds and star formation in clouds, respectively. We assume optically thick and thermalized CO emission, then L Õ CO(1≠0) = L Õ CO(2≠1) . We note that CO tend to become warm and optically thin as CO is exposed to the interstellar radiation in metal poor galaxies. [START_REF] Schruba | Low CO Luminosities in Dwarf Galaxies[END_REF] adopted R 21 = 0.7 which is the average value from HERACLES galaxies [START_REF] Rosolowsky | CO Line Ratios in Nearby Galaxies[END_REF] for the nearby dwarf galaxies. [START_REF] Saintonge | COLD GASS, an IRAM legacy survey of molecular gas in massive galaxies -II. The non-universality of the molecular gas depletion time-scale[END_REF] studied the 523 galaxies in xCOLD GASS survey and gave an average value of 0.79±0.03 with larger scatter at L Õ CO(1≠0) < 10 8 K km s ≠1 pc 2 . Therefore, the choice of R 21 may lead to the derived L Õ

CO(1≠0)

a factor of 1.4 different the one from our assumption. SFR is derived from the combination of far-UV and 24 µm emission for all the galaxies except the infrared bright galaxies for which the SFR is derived from the infrared luminosity.

Discussion

Conclusion

In this letter, we report an upper limit of CO J=2-1 in IZw18, using the observation from NOEMA Interferometer after its Phase II upgrade. We push down the detection limit of L Õ CO to be L Õ CO(2≠1) < 7.64 ◊ 10 3 K km s ≠1 pc ≠2 , ten times lower than that of [START_REF] Leroy | The Low CO Content of the Extremely Metal-poor Galaxy I Zw 18[END_REF]. As one of the most metal poor galaxy, IZw18 shows

L IR /L Õ CO(1≠0)
and SFR/L Õ CO(1≠0) ratios much higher than those of galaxies with higher metal abundance. Particularly, the SFR/L Õ CO(1≠0) constraints the CO-to-H 2 conversion factor to rise considerably at metallicity lower than 5% Z § . The high L [CII] /L CO(1≠0) ratio indicates a drastic change in the structure of the extremely metal-poor ISM of IZw18. An upper limit of the continuum emission at 1.3 mm is also obtained to constrain the Rayleigh Jeans tail of the SED and a submilimeter excess in IZw18 is excluded.

Conclusion

5

The SAMI Galaxy Survey: energy sources of the turbulent velocity dispersion in spatially-resolved local star-forming galaxies In this chapter, I investigate the energy sources of random turbulent motions of ionised gas from H-emission in eight local star-forming galaxies from the Sydney-AAO Multi-object Integral field spectrograph (SAMI) Galaxy Survey. These galaxies satisfy strict pure star-forming selection criteria to avoid contamination from active galactic nuclei (AGN) or strong shocks/outflows. Using the relatively high spatial and spectral resolution of SAMI, we find that -on sub-kpc scales our galaxies display a flat distribution of ionised gas velocity dispersion as a function of star formation rate (SFR) surface density. A major fraction of our SAMI galaxies shows higher velocity dispersion than predictions by feedbackdriven models, especially at the low SFR surface density end. Our results suggest that additional sources beyond star formation feedback contribute to driving random motions of the interstellar medium (ISM) in star-forming galaxies.

Introduction

The kinematics, structure and star formation activity of a galaxy depend on a combination of complex physical processes such as gravity, turbulence, magnetic fields, radiation, heating/cooling, feedback, accretion, operating both interior to and exterior to the galaxy. The relative importance of these processes is expected to depend on cosmic evolution and galactic environment. Galaxies at different cosmic epochs show quite distinct properties. Compared to their high-redshift counterparts at similar stellar masses, local star-forming galaxies are larger, and have relatively lower gas fractions and lower SFRs [START_REF] Leroy | The Molecular Interstellar Medium of Dwarf Galaxies on Kiloparsec Scales: A New Survey for CO in Northern, IRAS-detected Dwarf Galaxies[END_REF]Daddi et al., 2010a;[START_REF] Tacconi | High molecular gas fractions in normal massive star-forming galaxies in the young Universe[END_REF][START_REF] Madau | Cosmic Star-Formation History[END_REF]. They are also less likely to experience violent events such as major mergers and gas accretion [START_REF] Baugh | Evolution of the Hubble sequence in hierarchical models for galaxy formation[END_REF][START_REF] Genzel | From Rings to Bulges: Evidence for Rapid Secular Galaxy Evolution at z ˜2 from Integral Field Spectroscopy in the SINS Survey[END_REF][START_REF] Robotham | Galaxy And Mass Assembly (GAMA): galaxy close pairs, mergers and the future fate of stellar mass[END_REF].

Many theoretical and observational studies suggest that gas in higher redshift galaxies has larger random motions compared to gas in low-redshift galaxies [START_REF] Nesvadba | Extreme Gas Kinematics in the z=2.2 Powerful Radio Galaxy MRC 1138-262: Evidence for Efficient Active Galactic Nucleus Feedback in the Early Universe?[END_REF][START_REF] Lehnert | Physical Conditions in the Interstellar Medium of Intensely Star-Forming Galaxies at Redshift˜2[END_REF][START_REF] Lehnert | On the self-regulation of intense star-formation in galaxies at z = 1-3[END_REF][START_REF] Schreiber | The SINS Survey: SINFONI Integral Field Spectroscopy of z ˜2 Star-forming Galaxies[END_REF][START_REF] Wisnioski | The KMOS 3D Survey: Design, First Results, and the Evolution of Galaxy Kinematics from 0.7 = z = 2.7[END_REF]. These random, turbulent motions may play a crucial role in regulating the formation of stars [START_REF] Green | High star formation rates as the origin of turbulence in early and modern disk galaxies[END_REF][START_REF] Federrath | The Star Formation Rate of Turbulent Magnetized Clouds: Comparing Theory, Simulations, and Observations[END_REF][START_REF] Padoan | The Star Formation Rate of Molecular Clouds[END_REF]. However, the origin and energy source of the turbulence remains poorly understood. External mechanisms like gas accretion from the intergalactic medium and minor mergers [START_REF] Glazebrook | The Dawes Review 1: Kinematic Studies of Star-Forming Galaxies Across Cosmic Time[END_REF], and internal mechanisms such as star formation feedback (stellar winds, supernovae), cloud-cloud collisions in the disc [START_REF] Tasker | Star Formation in Disk Galaxies. I. Formation and Evolution of Giant Molecular Clouds via Gravitational Instability and Cloud Collisions[END_REF], the release of gravitational energy via accretion of cold gas streams from the halo or the inspiral of clumps [START_REF] Klessen | Accretion-driven turbulence as universal process: galaxies, molecular clouds, and protostellar disks[END_REF], galactic shear from the differential rotation in disc galaxies [START_REF] Krumholz | Is Turbulence in the Interstellar Medium Driven by Feedback or Gravity? An Observational Test[END_REF], spiral-arms shocks in spiral galaxies [START_REF] Tamburro | What is Driving the H I Velocity Dispersion?[END_REF] and others can poteintially drive such turbulence (Mac [START_REF] Low | Control of star formation by supersonic turbulence[END_REF][START_REF] Elmegreen | Star Formation in Disks: Spiral Arms, Turbulence, and Triggering Mechanisms[END_REF][START_REF] Federrath | The Link between Turbulence, Magnetic Fields, Filaments, and Star Formation in the Central Molecular Zone Cloud G0.253+0.016[END_REF][START_REF] Zhou | The SAMI Galaxy Survey: energy sources of the turbulent velocity dispersion in spatially resolved local star-forming galaxies[END_REF].

Several studies have been carried out to explore the energetic drivers of the turbulence in both high and low redshift disc galaxies, e.g., [START_REF] Elmegreen | Interstellar Turbulence I: Observations and Processes[END_REF], [START_REF] Scalo | Interstellar Turbulence II: Implications and Effects[END_REF], [START_REF] Tamburro | What is Driving the H I Velocity Dispersion?[END_REF][START_REF] Gritschneder | Driving Turbulence and Triggering Star Formation by Ionizing Radiation[END_REF]. [START_REF] Green | DYNAMO -I. A sample of H--luminous galaxies with resolved kinematics[END_REF] found that the gas velocity dispersion increases with SFR in star-forming galaxies both locally and at high redshift. Based on observations and analytic considerations, [START_REF] Lehnert | Physical Conditions in the Interstellar Medium of Intensely Star-Forming Galaxies at Redshift˜2[END_REF] and [START_REF] Lehnert | On the self-regulation of intense star-formation in galaxies at z = 1-3[END_REF] speculated that there is a relation between velocity dispersion and SFR surface density ( SFR ) in active star-forming galaxies at z ≥ 1-3, and that it is the intense star formation that supports the high velocity dispersion and thus balances the gravitational pressure. In contrast, [START_REF] Genzel | The Sins Survey of z ˜2 Galaxy Kinematics: Properties of the Giant Star-forming Clumps[END_REF] found that the velocity dispersion correlates only weakly with SFR in their study of giant star-forming clumps in five galaxies at z ≥ 2 together with other rotation-dominant star-forming galaxies, lensed galaxies, and dispersion-dominated galaxies at z ≥ 2. They suggest that a large-scale release of gravitational energy could induce the global large random motions in high-redshift galaxies, and that local star formation feedback triggering outflows and stirring up the interstellar medium (ISM) drives the local variation of turbulent, random motions.

Spatially resolved information is vital to understand the details of the physical processes that drive different interactions within galaxies. The three-dimensional (3D) spectra of galaxies uncover the distribution of the physical properties and give clues to how the internal physical processes shape the galaxies by connecting the spectral information with its position in the galaxy. Integral field spectroscopy (IFS) enables us to obtain this crucial spatial information. More importantly, IFS gives us both spectral and kinematic information; i.e., the intensity-weighted gas velocity and velocity dispersion along the line of sight. Taking advantage of this, IFS surveys such as the Calar Alto Legacy Integral Field Area Survey (CALIFA; [START_REF] Sánchez | CALIFA, the Calar Alto Legacy Integral Field Area survey. I. Survey presentation[END_REF], the SAMI Survey [START_REF] Croom | The Sydney-AAO Multi-object Integral field spectrograph[END_REF][START_REF] Bryant | The SAMI Galaxy Survey: instrument specification and target selection[END_REF], and the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) Survey [START_REF] Bundy | Overview of the SDSS-IV MaNGA Survey: Mapping nearby Galaxies at Apache Point Observatory[END_REF] have made significant progress in this area. SAMI has a higher instrumental resolution ‡ = 29 km s ≠1 at 6250 -7350 Å [START_REF] Sharp | The SAMI Galaxy Survey: cubism and covariance, putting round pegs into square holes[END_REF] than the other IFS surveys mentioned above, which have instrumental resolution > 80 km s ≠1 at a similar wavelength range [START_REF] Sánchez | IFUs surveys, a panoramic view of galaxy evolution[END_REF]. In this work we investigate the properties of the ISM in star-forming galaxies using data from the SAMI Galaxy Survey. We measure maps of SFR and gas velocity dispersion, and use these maps to derive the relation between SFR and gas velocity dispersion. We further include SFR and velocity dispersion data from the literature, and determine the dependence of velocity dispersion on redshift, up to z ≥ 3. Section 5.2 presents the sample selection criteria and data reduction including the signal-to-noise ratio criteria, the data source and their reduction strategy, and the estimation of the magnitude of beam smearing effect. In Section 5.3, our results and comparison with high-redshift and H-luminous local star-forming galaxies are presented. We further discuss the main source(s) of the turbulence in star-forming galaxies in Section 5.4. Section 5.5 summarzies the conclusions of this work. A standard cosmology of H 0 = 70 km s ≠1 Mpc ≠1 , m = 0.3, = 0.7 is assumed throughout.

Sample and data analysis

Sample selection

The SAMI Galaxy Survey

We use the SAMI Galaxy Survey [START_REF] Croom | The Sydney-AAO Multi-object Integral field spectrograph[END_REF] to select local star-forming galaxies. The SAMI Survey will observe in total ≥ 3400 galaxies. It covers a broad range of galaxies in stellar mass and environment. The sample targets redshifts 0.004 -0.095, Petrosian magnitudes r pet < 19.41 , stellar masses 10 7 -10 12 M § and environments from isolated field galaxies through group galaxies to cluster galaxies [START_REF] Bryant | The SAMI Galaxy Survey: instrument specification and target selection[END_REF][START_REF] Owers | The SAMI Galaxy Survey: the cluster redshift survey, target selection and cluster properties[END_REF].

SAMI is mounted at the prime focus on the Anglo-Australian Telescope. It has a 1 degree diameter field of view and uses 13 fused fibre bundles (Hexabundles, Bland-Hawthorn et al., 2011;[START_REF] Bryant | Focal ratio degradation in lightly fused hexabundles[END_REF] with a high (75%) fill factor. Each of the bundles contains 61 fibres of 1.6 arcsec diameter which results in diameter of 15 arcsec in each integral field unit (IFU). The IFUs, together with 26 sky fibres, are inserted into pre-drilled plates using magnetic connectors. SAMI fibres are fed to the double-beam AAOmega spectrograph [START_REF] Sharp | Performance of AAOmega: the AAT multi-purpose fiber-fed spectrograph[END_REF]. AAOmega provides a range of different resolutions and wavelength ranges. For the SAMI Galaxy Survey, the 570V grating at 3700 -5700 Å is used to give a resolution of R = 1730 ( ‡ = 74 km s ≠1 ), and the R1000 grating from 6250 -7350 Å for a resolution of R = 4500 ( ‡ = 29 km s ≠1 ) (Table 5.1) [START_REF] Sharp | The SAMI Galaxy Survey: cubism and covariance, putting round pegs into square holes[END_REF]. Therefore, each SAMI object has one blue and one red data cube. Early reduced datacubes are included in the first public data release (Allen et al., 2015, Green et al. in prep.). All datacubes are constructed on spatial grids where each grid cell has a size of 0.5 arcsec ◊0.5 arcsec corresponding to a physical scale of ≥ 0.5 ◊ 0.5 kpc 2 for our sample of local star-forming galaxies (z ≥ 0.05). Note that the spatial resolution of SAMI data is determined by the average seeing in observations (≥ 2.5arcsec), corresponding to a physical scale of 2.5 kpc (z ≥ 0.05).

The spectral fitting pipeline of SAMI galaxies, LZIFU [START_REF] Ho | The SAMI Galaxy Survey: shocks and outflows in a normal star-forming galaxy[END_REF]Ho et al., 2016), is designed to extract two-dimensional emission line flux maps and kinematic maps to investigate the dynamics of gas in galaxies. LZIFU uses up to three Gaussian profiles to fit emission lines, separating up to three different kinematic components contributing to the emission lines.

Maps of SFR and SFR surface density [START_REF] Medling | The SAMI Galaxy Survey: spatially resolving the main sequence of star formation[END_REF] are also available in the SAMI database. Briefly, these maps are calculated from extinction-corrected H-flux maps using the calibration in . Middle: ionised gas velocity in km s ≠1 . Bottom: ionised gas velocity dispersion in km s ≠1 . In all images, we only display spaxels with S/N > 34 (see Section 5.2.2.2 for the choice of S/N). The red crosses are the centres of the galaxies. The white solid lines are the major axes (see Section 5.2.2.1).

Tab. 5.2: Properties of the eight star-forming galaxies in our final sample of star-forming SAMI galaxies. [START_REF] Taylor | Galaxy And Mass Assembly (GAMA): stellar mass estimates[END_REF]). 2 Effective radius, i.e., half light radius, also from the GAMA survey [START_REF] Kelvin | Galaxy And Mass Assembly (GAMA): Structural Investigation of Galaxies via Model Analysis[END_REF]. 3 Ellipticity is from the GAMA survey (http://www.gama-survey.org/dr2/tools/sov.php). We use the GAL_ELLIP_R to get the R-band axis ratio. The relation between minor-to-major axis ratio and ellipticity is: b/a = 1 -ellipticity. 4 Inclination angle. The calculation is based on classical Hubble formula:

cos 2 i = ((b/a) 2 ≠ q 2 0 )/(1 ≠ q 2 0 )) 1/2
, where b/a is the minor-to-major axis ratio, i is the inclination angle and q 0 = 0.2 (i = 90°for b/a < q 0 ). 5 Flux weighted global gas velocity dispersion. Only the pixels with ‡gas > 2 v grad are considered (see more in Section 5.2.2.3). 6 Flux weighted SFR surface density. Only the pixels with ‡gas > 2 v grad are considered (see more in Section 5.2.2.3). [START_REF] Kennicutt | Past and future star formation in disk galaxies[END_REF]. Extinction corrections are calculated using the Balmer decrement, and flux is converted to luminosity using distances calculated from the flow-corrected redshifts of the GAMA Survey Catalog [START_REF] Baldry | Galaxy And Mass Assembly (GAMA): the galaxy stellar mass function at z 0.06[END_REF].

Our sample

To select star-forming galaxies from the parent SAMI sample, we use optical emission line diagnostic diagrams, so-called 'BPT/VO' diagrams [START_REF] Baldwin | Classification parameters for the emission-line spectra of extragalactic objects[END_REF][START_REF] Veilleux | Spectral classification of emission-line galaxies[END_REF] with multicomponent emission line fits. In the following analysis of the chosen star-forming galaxies, we use single component fits, which are sufficient to describe star-forming galaxies. In the most conservative way, we select those galaxies with all detected spaxels lying below the theoretical extreme starburst lines in all the three BPT/VO87 diagrams, i.e., [NII]⁄6583/H-vs. [OIII]⁄5007/H-, [SII]⁄6717,⁄6731/H-vs.

[OI]⁄6300/H-and [NII]⁄6583/H-vs. [OIII]⁄5007/H- [START_REF] Baldwin | Classification parameters for the emission-line spectra of extragalactic objects[END_REF][START_REF] Veilleux | Spectral classification of emission-line galaxies[END_REF][START_REF] Kewley | Theoretical Modeling of Starburst Galaxies[END_REF]. Thus we minimise the contamination of AGNs, outflows, and shock contamination. We note that there are a few spaxels with elevated non-thermal-ratios suggestive of shocks in 508421. Because these spaxels are few in numbers and lie below the theoretical extreme starburst lines in all the three BPT/VO diagrams, we still keep this galaxy.

We find that eight out of 756 SAMI galaxies satisfy our complete selection criteria. By the time of this work, 756 of the 3400 SAMI galaxies have had reduced data cubes available with the required analysis products. We find 22 star-forming galaxies meeting the criteria, but 11 of them are cluster galaxies which may be influenced by their environments. We choose not to discuss cluster galaxies, in order to focus on turbulence induced locally through accretion and/or galaxy-internal processes such as star formation feedback. Among the remaining 11 galaxies, two lack the information of stellar velocity dispersion (van de Sande et al., 2017, this will be relevant for a follow-up paper to determine the Toomre Q and that we want to use the same set of eight galaxies for this and the follow-up paper) and another lacks enough spaxels with high enough signal-to-noise ratio. Table 6.1 lists the basic information of the eight galaxies in our final sample. Our eight star-forming galaxies are at redshifts ranging from 0.017 to 0.055, most of them at the high end. Their stellar masses range from 6.3 ◊10 9 5.2 Sample and data analysis M § to 6.2 ◊10 10 M § and the median stellar mass is 2.5 ◊10 10 M § , similar to most of the galaxies in SAMI sample [START_REF] Bryant | The SAMI Galaxy Survey: instrument specification and target selection[END_REF].

Gas kinematic information

Definition of the major axis

To define the major axis, we first define the centre of each galaxy as the centre of each datacube. We note that this is consistent with the photometric and kinematic centres of each galaxy in our sample. The major axis of each galaxy is determined based on the velocity field of the galaxy. The centre velocity (v centre ) of each galaxy is measured by averaging the central four pixels; gas velocity is given as:

v gas = v ≠ v centre .

Velocity and Velocity dispersion

Ionised gas velocity (v gas ) and gas velocity dispersion ( ‡ gas ) are measured from the emission lines by the LZIFU pipeline. The velocity dispersions in the datacubes have removed instrument resolutions, i.e., ‡ gas = ( ‡ 2 obs ≠ ‡ 2 instr ) 1/2 , where ‡ instr is the instrumental velocity dispersion and ‡ obs is the observed velocity dispersion). We emphasise that the error of the line width can be underestimated because of the limitation of the instrument resolution. Thus we make an estimation of the lower limit of the reliable velocity dispersion. Given that ‡ instr = 29 km s ≠1 (assuming the instrument resolution for the H-line is fixed and exactly 29 km s ≠1 ), if we want to resolve an intrinsic velocity dispersion of 12 km s ≠1 with signal to noise ratio (S/N) of 5 (i.e., (S/N) true © ‡gas d ‡gas = 5, ‡ gas is the intrinsic velocity dispersion in the datacubes), then we can derive: ( ‡ 2 instr / ‡ 2 gas + 1). Therefore, in order to resolve velocity dispersion down to 12 km s ≠1 (corresponding to the thermal broadening velocity of ionised gas) with S/N > 5, we select only spaxels with S/N > 34 for velocity dispersion. Given that the velocity dispersion we are measuring is lower than the spectral resolution, we do simple Monte Carlo simulations to see if there is a systematic overestimation. The simulations test (S/N) obs from 10 to 34 and a range of line widths between 5 km s ≠1 and 40 km s ≠1 . They confirm the scaling predicted by our very simple analytic model: indeed, in order to measure ‡ gas (with a target intrinsic S/N) one needs a much larger S/N on the observed. Assuming a velocity sampling of 5 km/s, we obtain an excellent fit to our simple analytic formula above, i.e., a (S/N) obs > 34 is required for 12 km s ≠1 with (S/N) true > 5. However, the absolute S/N requirement eventually depends on the specifics of the instrument, and a separate publication fully dedicated to these Monte Carlo simulations is in preparation.

‡ 2 gas = ‡ 2 obs ≠ ‡ 2 instr ; d ‡ 2 gas = d ‡ 2 obs ≠ d ‡ 2 instr , (d ‡ 2 instr = 2d ‡ instr = 0 , ‡ instr =
Maps of gas velocity (v gas ) and gas velocity dispersion ( ‡ gas ) together with SFR are shown in Figure 5.1.

Beam smearing e ect

The measured velocity dispersion may be overestimated in the presence of a velocity gradient within the scale of one beam; this is what we call beam smearing. The magnitude of beam smearing increases

Sample and data analysis

with increasing local velocity gradient. We use the velocity gradient (v grad ) across a spaxel [START_REF] Varidel | Resolved Gas Kinematics in a Sample of Low-Redshift High Star-Formation Rate Galaxies[END_REF],

v grad =  (v[x + 1, y] ≠ v[x ≠ 1, y]) 2 + (v[x, y + 1] ≠ v[x, y ≠ 1]) 2 . (5.1)
to estimate the magnitude of the beam smearing (x, y are the image grid cell indices). We use the upper, lower, left and right neighbours of each spaxel to measure v grad . These 5 pixels make up the central cross in a 3 ◊ 3 binned spaxel, which is roughly equivalent to the size of the seeing limited FWHM of ≥2.5arcsec. Therefore, v grad is indicative of the beam smearing effect within the size of the beam FWHM. A higher v grad can indicate a larger beam smearing effect and thus a higher observed velocity dispersion. In order to account for the effect of beam smearing in the following analysis, we remove the spaxels whose line-width may have been primarily caused by a velocity gradient due to beam smearing. Note that the boundary spaxels don't have enough necessary neighbouring spaxels to calculate velocity gradient, we choose to also remove boundary spaxels.

In Figure 5.2, we show the measured velocity dispersion as a function of the velocity gradient for each spaxel and galaxy. We tried three different criteria to account for the beam smearing effect. We exclude spaxels with:

1. v grad > 0.5 ‡ gas (solid line), 2. v grad > ‡ gas (dashed line),

3. v grad > 20 km s ≠1 (dotted line).

We find that our results do not depend on the particular choice of the beam-smearing cut. All three selection criteria yield consistent results. A flat but elevated distribution of gas velocity dispersion (see more in Section 5.3.2.1) is shown in all of the three cases. We choose to preserve only the spaxels with ‡ gas > 2 v grad (those above the black solid line) in the following analysis. Our method here is similar to the simple analytic calculation in Bassett et al. (2014, equation (1)).

Spatial resolution

We compare our sample with high redshift surveys and local H-luminous galaxies. The data in [START_REF] Lehnert | On the self-regulation of intense star-formation in galaxies at z = 1-3[END_REF] has FWHM ≥0.6arcsec and pixel scale 0.25arcsec corresponding to 5 kpc and 2 kpc at z ≥ 2. The seeing limit of our sample is ≥ 2.5arcsec and the pixel scale is 0.5arcsec, which correspond to 2.5 kpc and 0.5 kpc at z ≥ 0.05. [START_REF] Genzel | Strongly baryon-dominated disk galaxies at the peak of galaxy formation ten billion years ago[END_REF] has FWHM up to 0.2arcsec corresponding to ≥ 1.5 kpc at z ≥ 0.76 -2.65. [START_REF] Green | DYNAMO -I. A sample of H--luminous galaxies with resolved kinematics[END_REF] has spatial resolutions of 1 -3 kpc. We reach similar resolutions as [START_REF] Genzel | The Sins Survey of z ˜2 Galaxy Kinematics: Properties of the Giant Star-forming Clumps[END_REF] and [START_REF] Green | DYNAMO -I. A sample of H--luminous galaxies with resolved kinematics[END_REF] and better than [START_REF] Lehnert | On the self-regulation of intense star-formation in galaxies at z = 1-3[END_REF]. All the works above either construct models or use simulations to remove the beam smearing effect.

Results

We investigate the relation between SFR and ‡ gas spaxel by spaxel (locally), and within individual galaxies (globally), to see if star formation drives the velocity dispersion in these local star-forming galaxies.

5.3.1

The spatial distribution of SFR , v gas , and ‡ gas

In Figure 5.1, we show the maps of SFR , gas velocity and gas velocity dispersion for each of our eight galaxies. The major axes and galaxy centres are labeled in the images. We see:

a) The SFR maps have various distributions. There are often multiple peaks and rings, and some of the peaks are not at the centre, indicating local structures such as spiral arms, star-forming clumps, etc.

b) All galaxies show clear velocity gradients indicating rotation.

c) All galaxies show a gas velocity dispersion peak at the centre.

d) The distribution of gas velocity dispersion does not always follow the distribution of SFR (i.e. the peaks in ‡ gas do not always correlate with those in SFR ). This is self-consistent because regions of intense mechanical or radiative energy injections (e.g., star-forming regions) are over-pressured and compact while over-pressurized gas is over a much larger scale.

5.3.2

The ‡ gas -SFR relation in local and high redshift star-forming galaxies analysis

After removing the spaxels with high v grad (keeping those with ‡ gas > 2 v grad ), we compare the spatially resolved relation between ‡ gas and SFR in our local galaxies with that in high redshift galaxies [START_REF] Lehnert | Physical Conditions in the Interstellar Medium of Intensely Star-Forming Galaxies at Redshift˜2[END_REF] in Figure 5.3. We also insert a zoom-in of our SAMI data with a logarithmic y-axis.

As shown by the filled circles, the velocity dispersion of our local star-forming galaxies is almost constant around 20 km s ≠1 , covering a SFR range of over an order of magnitude. As discussed in Section 5.2.2.2, SAMI is able to detect ‡ gas below 15 km s ≠1 for a sufficiently high S/N (here 34), therefore the cut-off at ≥15 km s ≠1 represents a physical cut-off. The flat and tight distribution of ‡ gas in our local star-forming galaxies does not present an obvious correlation with SFR . There are some outliers with ‡ gas > 40 km s ≠1 that may result from a shocked component in 508421 as mentioned in Section 5.2.1.2 and 5.2.2.3. We also connect the behaviour of the gas in star-forming galaxies from low-z to high-z by superimposing our data onto the results in Figure 12 from [START_REF] Lehnert | Physical Conditions in the Interstellar Medium of Intensely Star-Forming Galaxies at Redshift˜2[END_REF]. The crosses and asterisks in different colours refer to the 13 actively star-forming galaxies at z ≥ 2 in [START_REF] Lehnert | Physical Conditions in the Interstellar Medium of Intensely Star-Forming Galaxies at Redshift˜2[END_REF]. The high redshift galaxies in [START_REF] Lehnert | Physical Conditions in the Interstellar Medium of Intensely Star-Forming Galaxies at Redshift˜2[END_REF] show higher velocity dispersion with larger scatter than our local star-forming galaxies. They are also more or less positively correlated with SFR , however, with substantial scatter. This may indicate that star formation feedback plays a more important role in high redshift star-forming galaxies than in their local counterparts. High redshift star-forming galaxies, as shown in [START_REF] Genzel | The Sins Survey of z ˜2 Galaxy Kinematics: Properties of the Giant Star-forming Clumps[END_REF][START_REF] Genzel | The SINS/zC-SINF Survey of z ˜2 Galaxy Kinematics: Evidence for Gravitational Quenching[END_REF], are mostly irregular and clumpy while local galaxies are stable and rotationally supported, as seen in Figure 5.1. Therefore, juvenile high redshift galaxies could be more easily affected by their intense star formation activity and mature local galaxies could be more sensitive to galactic-scale dynamics like cloud-cloud collisions, galactic shear and self-gravity than to star formation feedback. We discuss this further in Section 5.4.

Results

Theoretical models for ‡ gas

The curves in Figure 5.3 denote different models proposed by [START_REF] Lehnert | Physical Conditions in the Interstellar Medium of Intensely Star-Forming Galaxies at Redshift˜2[END_REF] to explain the relation between ‡ gas and SFR .

If the dissipated energy comes from star formation, and star formation induces high pressures, then a simple scaling relation is expected. This is indicated by the solid black curves, in the form ‡ Ã (' Ė) 1/2 , where Ė is the energy injection rate due to star formation, and ' is the coupling efficiency between the injected energy and the ISM. According to [START_REF] Dib | The Supernova Rate-Velocity Dispersion Relation in the Interstellar Medium[END_REF], when modelling the ISM with a coupling efficiency of 25% (a conservative value), quiescent galaxies may switch to a starburst mode at SFR = 10 ≠2.5 to 10 ≠2 M § yr ≠1 kpc ≠2 . The bottom two black solid lines are derived from such models using these two values, showing ‡ gas = 100

1/2 SFR and ‡ gas = 140

1/2 SFR ( ‡ gas is in km s ≠1 , SFR is in M § yr ≠1 kpc ≠2 , the same below). The third curve at the top shows ‡ gas = 240 1/2 SFR , using coupling efficiencies of 100% (an extreme and unrealistic value).

If the energy is dissipated through incompressible turbulence, another scaling relation would be expected. This is shown by the dashed curves in the form of ‡ gas à (' Ė) 1/3 where Ė is the energy dissipated through turbulence. The two black dashed curves show ‡ gas = 80 1/3 SFR and ‡ gas = 130 1/3 SFR , using coupling efficiencies of 25%, 100%, and a primary injection scale of 1 kpc.

If the turbulence is powered by gravity, assuming a simple Jeans relationship between mass and velocity dispersion, [START_REF] Lehnert | Physical Conditions in the Interstellar Medium of Intensely Star-Forming Galaxies at Redshift˜2[END_REF] derived the relation in the form of

‡ gas ≥ M 1/4 J G 1/2 1/4 gas = 54 km s ≠1 M 1/4 J,9 0.18
SFR , where G is the gravitational constant, gas is the gas mass surface density in M § pc ≠2 , M J,9 is the Jeans mass in units of 10 9 M § and SFR is in M § kpc ≠2 yr ≠1 . The red solid curve shows the velocity dispersion as a function of SFR of a 10 8 M § giant molecular cloud (GMC). Given the mass of our local star-forming galaxies are similar to the Milky Way, there is not any molecular cloud more massive than 10 8 M § [START_REF] Roman-Duval | Physical Properties and Galactic Distribution of Molecular Clouds Identified in the Galactic Ring Survey[END_REF], which means that the red curve represents an upper limit for the velocity dispersion obtained via the Jeans relation proposed by [START_REF] Lehnert | Physical Conditions in the Interstellar Medium of Intensely Star-Forming Galaxies at Redshift˜2[END_REF] Due to a characteristic temperature of 10 4 K [START_REF] Andrews | The Mass-Metallicity Relation with the Direct Method on Stacked Spectra of SDSS Galaxies[END_REF], the H-emission line has a typical thermal broadening of ≥ 12 km s ≠1 . In addition, the temperature distribution in an HII region is

Results

not uniform, so we estimate a maximum range of 10 -15 km s ≠1 [START_REF] Andrews | The Mass-Metallicity Relation with the Direct Method on Stacked Spectra of SDSS Galaxies[END_REF]. Here in all of the models we include the intrinsic thermal broadening of 12 km s ≠1 , but we also display the maximum range with the error-bars in the zoom-in figure .   None of the models can properly explain our local star-forming galaxies. ‡ Ã (' Ė) 1/2 and ‡ Ã (' Ė) 1/3 are consistent with some of the spaxels in our SAMI galaxies. However, all of the relations predict lower velocity dispersion than seen in a significant number of spaxels. Most of the data points of our galaxies are significantly above the bottom two black solid lines and the bottom black dashed line, which correspond to a realistic coupling efficiency of 25% [START_REF] Dib | The Supernova Rate-Velocity Dispersion Relation in the Interstellar Medium[END_REF] in the two models. Even if we consider the most extreme (and unrealistic) cases with coupling efficiencies of 100%, as shown by the top black solid line and the top black dashed line, the datapoints at the lower SFR end cannot be well explained. The same is true with the Jeans instability relation. Even if we assume extreme GMC masses as high as as 10 8 M § , it still underestimates the velocity dispersion of our galaxies. Moreover, the error bars in the zoom-in figure indicate that the uncertainty induced by the thermal broadening has minor influence on the distribution of the models. Therefore, simply considering energy injection from star formation, dissipation of incompressible turbulence, or release of gravitational energy alone as the source of velocity dispersion is not enough to explain the distribution of gas velocity dispersion in local star-forming galaxies.

Global (galaxy-averaged) analysis

In Figure 5.4, we look at the global behaviour of the gas in star-forming galaxies. We include the local H-luminous galaxies from [START_REF] Green | DYNAMO -I. A sample of H--luminous galaxies with resolved kinematics[END_REF] as green diamonds, and the z > 1 star-forming galaxies and star-forming clumps with good data quality 2 , for comparison. The area used to calculate the SFR of galaxies in [START_REF] Green | DYNAMO -I. A sample of H--luminous galaxies with resolved kinematics[END_REF] are from the radii based on an exponential model fit. Given their various sizes, they span a very large range of SFR . The global SFR s and velocity dispersions of our eight galaxies are listed in Table 6.1. The global SFR s and velocity dispersions are the flux weighted averages of spaxels within individual galaxies. Only the spaxels with ‡ gas > 2 v grad are considered in the measurement. All these works derive velocity dispersion from the H-emission lines. The grey contour denotes the distribution of local star forming galaxies with ‡ gas derived from HI [START_REF] Dib | The Supernova Rate-Velocity Dispersion Relation in the Interstellar Medium[END_REF], and we include the intrinsic thermal broadening of 12 km s ≠1 here. The difference in velocity dispersions between HI and H-comes from the thermal broadening of warm ionised gas at ≥ 10 4 K.

Comparing our eight SAMI galaxies to the high redshift galaxies, we see similar behaviour to the spatially resolved data (c.f. Figure 5.3). When taking the 67 H-luminous galaxies from [START_REF] Green | DYNAMO -I. A sample of H--luminous galaxies with resolved kinematics[END_REF] into account, we find that it can well connect the local and high-redshift galaxies. In Figure 5.4, we see that their distribution at the lower SFR end agrees with the distribution of our eight galaxies and the higher SFR end follows the distribution of the high redshift galaxies as well. [START_REF] Green | DYNAMO -I. A sample of H--luminous galaxies with resolved kinematics[END_REF]'s local star-forming galaxies are chosen to have similar properties to the high redshift galaxies, such as high gas fraction and high H-luminosities ( & 10 42 erg s ≠1 ). Note that we calculate the SFR of 2014) and z > 1 star-forming galaxies and clumps (see Section 5.3.2.2 for further details). Each blue (red) filled diamond shows one entire galaxy in our sample including (excluding) the contribution from thermal broadening ( ‡ thermal ≥ 12 km s ≠1 , [START_REF] Glazebrook | The Dawes Review 1: Kinematic Studies of Star-Forming Galaxies Across Cosmic Time[END_REF]. For the measurement of ‡gas and SFR , see footnotes in Table 6.1. Green diamonds refer to the H-luminous galaxies in [START_REF] Green | DYNAMO -I. A sample of H--luminous galaxies with resolved kinematics[END_REF]. The black filled squares, dark green triangles, red open squares and blue open circles refer to the z > 1 star-forming galaxies and clumps. The grey contour denotes the distribution of local star-forming galaxies with gas velocity dispersion derived from HI [START_REF] Dib | The Supernova Rate-Velocity Dispersion Relation in the Interstellar Medium[END_REF], and we include the intrinsic thermal broadening of 12 km s ≠1 here.

Tab. 5.3: Spearman correlation coefficient (rs). ‡ gas vs. SFR r s P ú SAMI + Green † 0.72 π0.01 SAMI + high-z ‡ 0.53 π0.01 SAMI + Green † + high-z ‡ 0.76 π0.01 galaxies in [START_REF] Green | DYNAMO -I. A sample of H--luminous galaxies with resolved kinematics[END_REF] without flux weighting, so the SFR may be underestimated compared to other galaxies.

We measure the Spearman correlation coefficients between gas velocity dispersion and SFR , for three different groups of the three sets of data. The results are listed in Table 5.3. Our eight SAMI galaxies together with [START_REF] Green | DYNAMO -I. A sample of H--luminous galaxies with resolved kinematics[END_REF]'s H-luminous galaxies show strong correlation between velocity dispersion and SFR (r s = 0.72). When we include the high-z galaxies, the correlations become a bit stronger (r s = 0.76). However, when combining our eight SAMI galaxies with only the high-z galaxies, the correlation becomes moderate (r s = 0.53). Note that the global SFR are flux weighted. Therefore this result comes from relatively more active star-forming regions, which is not contrary to the conclusion we draw from the spatially resolved analysis in Section 5.3.2.1. Stellar feedback alone is insufficient to drive the observed ‡ gas , especially at low SFR surface density (Section 5.3.2.1), while it can become dominant when SFR surface density is high enough (Section 5.3.2.2).

Discussion

Main driver(s) of velocity dispersion

As mentioned in Section 5.3.2.1, the flat and elevated (compared to the model predictions) distribution of spaxels of our SAMI galaxies shown in Figure 5.3 indicates that the star formation feedback is unlikely to dominate the gas velocity dispersion at the scale of ≥ 0.5 ◊ 0.5 kpc 2 . Thus, additional drivers of turbulence must be acting in these local, low SFR galaxies. As indicated by the flatness of the distribution, such sources need to be common among galaxies and not vary much within galaxies.

The drivers of turbulence can either compress the gas (compression processes) or directly excite solenoidal motions of gas (stirring processes) [START_REF] Federrath | The Link between Turbulence, Magnetic Fields, Filaments, and Star Formation in the Central Molecular Zone Cloud G0.253+0.016[END_REF][START_REF] Zhou | The SAMI Galaxy Survey: energy sources of the turbulent velocity dispersion in spatially resolved local star-forming galaxies[END_REF].

Stellar feedback like stellar winds of OB stars and Wolf Rayet stars, supernova explosions, as well as accretion processes (such as accretion onto a galaxy) and gravitational contraction are able to compression the gas, and then increase gas velocity dispersion and induce star formation at the same time. Therefore, SFR (or SFR ) may not be directly related to gas velocity dispersion. Both SFR (or SFR ) and gas velocity dispersion can be affected simultaneously by the same sources (e.g. accretion, gravity etc.), but to different degree. Krumholz and Burkhart (2016) proposed that the turbulence in the ISM is driven by gravity rather than stellar feedback. The higher (than expected) velocity dispersion in Figure 5.3 may be due to the release of gravitational energy. However, Krumholz and Burkhart (2016)'s conclusion comes from the constraint of rapid star-forming, high velocity dispersion galaxies. Their models do not show any apparent difference at low velocity dispersion. Moreover, they investigate the relation with SFR rather than SFR and on entire galaxies rather than on spatially resolved regions. On the other hand, [START_REF] Lehnert | Physical Conditions in the Interstellar Medium of Intensely Star-Forming Galaxies at Redshift˜2[END_REF]'s models in Figure 5.3 reveal that -(i) self-gravity is not sufficient even if the model (red solid curve) adopts an extremely massive the star-forming clump as 10 8 M § , which is too big for local star-forming galaxies; (ii) turbulence can be driven by the bulk motions induced by energy injection from star formation, and then cascade, redistribute and dissipate the energy down to smallest scales. Star formation powers the turbulence of galaxies with high velocity dispersions in complex ways, but star formation alone is insufficient to explain the gas velocity dispersions in our local star-forming galaxies. Generally, both gravity and star formation can power the turbulence but dominate at different redshifts and/or in different environments.

Stirring processes like galactic shear, jets and/or outflows can induce solenoidal motions, i.e., increase the velocity dispersion, but suppress the star formation. Shear is a typical driver of the turbulence in the centres of our galaxy and possibly other galaxies [START_REF] Krumholz | A dynamical model for the formation of gas rings and episodic starbursts near galactic centres[END_REF][START_REF] Kruijssen | Towards a multi-scale understanding of the gas-star formation cycle in the Central Molecular Zone[END_REF][START_REF] Federrath | The Link between Turbulence, Magnetic Fields, Filaments, and Star Formation in the Central Molecular Zone Cloud G0.253+0.016[END_REF][START_REF] Zhou | The SAMI Galaxy Survey: energy sources of the turbulent velocity dispersion in spatially resolved local star-forming galaxies[END_REF]. Compared to compressive stirring mechanisms, solenoidal stirring processes have less influence on the density distribution [START_REF] Federrath | The Density Probability Distribution in Compressible Isothermal Turbulence: Solenoidal versus Compressive Forcing[END_REF][START_REF] Federrath | Comparing the statistics of interstellar turbulence in simulations and observations. Solenoidal versus compressive turbulence forcing[END_REF]. Therefore, solenoidal driving mechanisms reduce the SFR compared to compressive sources of ‡ gas [START_REF] Federrath | The Star Formation Rate of Turbulent Magnetized Clouds: Comparing Theory, Simulations, and Observations[END_REF]. Solenoidal drivers (such as shear) may be able to provide an explanation for the distribution of our SAMI galaxies in Figure 5.3: suppressed SFR but relatively higher gas velocity dispersion than expected from star formation feedback.

Note that the velocity modes (solenoidal and compressible) do not grow independently of one another (for a detailed analysis in the case of compressive and solenoidal driving of the turbulence, see Federrath et al. (2010, Fig. 14) and Federrath et al. (2011, Fig. 3 bottom panel)). However, what we are referring to here when we talk about solenoidal and compressive modes, are the modes in the acceleration field (not the resulting velocity field) that drives the turbulence. A summary of the differences and implications of solenoidally-and compressively-driven turbulence and their implications for star formation is presented in [START_REF] Zhou | The SAMI Galaxy Survey: energy sources of the turbulent velocity dispersion in spatially resolved local star-forming galaxies[END_REF] and the main theoretical framework as well as comparison to observations are presented in [START_REF] Federrath | The Star Formation Rate of Turbulent Magnetized Clouds: Comparing Theory, Simulations, and Observations[END_REF].

Caveats

The medium being observed

H-emission traces ionized gas and gives information on the turbulences in HII regions. We may miss out the tracers showing the star formation feedback and effects of turbulence driven by galaxy dynamics (such as shear) by just analysing the ionized gas (and not including the atomic and molecular phases). Studies such as [START_REF] Stilp | Drivers of H I Turbulence in Dwarf Galaxies[END_REF] find two component fits are necessary for HI lines in nearby galaxies, and the broad components are mainly related to the star-formation intensity, which support the idea of star formation supporting the turbulence and gravitational instability/shear. Here we are limited to strong emission line data, but we have applied for ALMA time to follow-up a subset of our galaxies to study the velocity dispersion in the cold/molecular gas as well.

Removing the inner regions of the galaxies

In this paper, circum-nuclear regions of our galaxies are removed because of the strong influence of beam-smearing. However, circum-nuclear regions are very interesting and important in order to develop a complete picture of turbulence injection across a galaxy. The influence of the rotation curve could be especially important in circum-nuclear regions, providing a strong test of the gravitational shear/instability arguments. The broadest molecular lines are also found in the circum-nuclear gas of galaxies suggesting that star formation is playing a crucial role there (e.g. [START_REF] Wilson | The JCMT Nearby Galaxies Legacy Survey -IV. Velocity dispersions in the molecular interstellar medium in spiral galaxies[END_REF]. Here we chose to exclude the inner regions, where beam smearing significantly affects our data. Future studies that attempt to correct for beam smearing will be needed to investigate the turbulence in the circum-nuclear regions in detail.

Removing the galaxies that show any evidence for shocks

Shock driven turbulence contains important information about the connection between star formation feedback and turbulence. However, it is difficult to disentangle the contribution of star formation from possible AGN activity to extract actual SFRs accurately from the shock-heated gas. We would likely overestimate SFRs and thus overestimate the effect of star formation feedback. Thus, in a conservative way, we chose not to include any galaxies with signatures of shock-excited emission and focus on purely star-forming galaxies.

Dependence of velocity dispersion on rotational velocity

Figure 5.5 shows the dependence of velocity dispersion on rotational velocity. Overall, we see no strong dependence. However, we see some trends of decreasing velocity dispersion with increasing velocity, especially along the major axes (labeled with errorbars). The beam smearing effect would be higher in the center than the outside. For our galaxies, we have already removed the highly affected spaxels, thus the circum-nuclear regions are removed (Section 5.4.2.2).

conclusion

Random, turbulent motions are important in regulating the star formation in the galaxies, but the energy source of turbulent motions remains unclear. We investigate the random motions of the ionised gas in local star-forming galaxies. After very strict selection to avoid the possible contamination by AGN, shocks and outflows, we find eight SAMI galaxies satisfying the pure star-forming criteria based on emission line diagnostic diagrams. We minimise the influence of beam smearing by removing the spaxels with ‡ gas < 2 v grad before further analysis (Figure 5.2). The spatially resolved images and high spectral resolution of the SAMI Galaxy Survey shows that, on scales of 0.5 ◊ 0.5 kpc 2 , turbulence within local star-forming galaxies is not exclusively driven by star formation feedback. The flat but elevated (compared to model predictions) distribution of gas velocity dispersion as a function of SFR (Figure 5.3) implies that there must be some additional energy source(s) besides star formation feedback, especially at the low SFR end. Such source(s) need to be common among local star-forming galaxies and do not vary spatially across galaxies. The difference with the high-redshift galaxies and H-luminous galaxies (Figures 5.3 and 5.4) indicates that the low SFR in these local galaxies is too weak to explain the random motions of the ionised gas. Juvenile high redshift galaxies could be more sensitive to their intense star formation activity while mature local galaxies could be more influenced by galactic-scale dynamics like gravity, and galactic shear than by star formation feedback.

conclusion 6

Spatially resolved dust emission of extremely metal poor galaxies 1

In this chapter, I present infrared (IR) spectral energy distributions (SEDs) of individual star-forming regions in four extremely metal poor (EMP) galaxies with metallicity Z . Z § /10 as observed by the Herschel Space Observatory. With the good wavelength coverage of the SED, it is found that these EMP star-forming regions show distinct SED shapes as compared to those of grand design Spirals and higher metallicity dwarfs: they have on average much higher f 70µm /f 160µm ratios at a given f 160µm /f 250µm ratio; single modified black-body (MBB) fittings to the SED at ⁄ Ø 100 µm still reveal higher dust temperatures and lower emissivity indices compared to that of Spirals, while two MBB fittings to the full SED with a fixed emissivity index (-= 2) show that even at 100 µm about half of the emission comes from warm (50 K) dust, in contrast to the cold (≥20 K) dust component. Our spatially resolved images further reveal that the far-IR colors including f 70µm /f 160µm , f 160µm /f 250µm and f 250µm /f 350µm are all related to the surface densities of young stars as traced by far-UV, 24 µm and SFRs, but not to the stellar mass surface densities. This suggests that the dust emitting at wavelengths from 70 µm to 350 µm is primarily heated by radiation from young stars.

Introduction

Stars born within primordial galaxies in the early universe form out of gas with no or little metals. These distant galaxies are, however, difficult to detect. Nearby extremely metal poor (EMP) starforming galaxies are chemically unevolved, and thus offer important astrophysical laboratories for our understandings of the interstellar medium (ISM) and star formation in the low metallicity environments [START_REF] Kunth | The most metal-poor galaxies[END_REF][START_REF] Rémy-Ruyer | Revealing the cold dust in low-metallicity environments. I. Photometry analysis of the Dwarf Galaxy Survey with Herschel[END_REF][START_REF] Shi | Inefficient star formation in extremely metal poor galaxies[END_REF]Shi et al., 2015b). Dust grains play a vital role in galaxy formation and evolution. Characterizing the dust properties by investigating the infrared (IR) emission is a powerful way to understanding the ISM and evolution of dwarf galaxies [START_REF] Feldmann | The equilibrium view on dust and metals in galaxies: Galactic outflows drive low dust-to-metal ratios in dwarf galaxies[END_REF].

The integrated dust properties of dwarfs have been investigated with Spitzer Space Telescope. It is found that aromatic features are absent in the metal poor galaxies. [START_REF] Engelbracht | Metallicity Effects on Mid-Infrared Colors and the 8 µm PAH Emission in Galaxies[END_REF] pointed out that the ratio f 8µm /f 24µm depends strongly on the metallicity (where f ⁄ is the flux density at wavelength ⁄), with significantly lower values (mean ratio 0.08 ± 0.04) for galaxies below one-third of the Solar metallicity than those at higher metallicities (mean ratio 0.70 ± 0.53).2 [START_REF] Draine | Infrared Emission from Interstellar Dust. IV. The Silicate-Graphite-PAH Model in the Post-Spitzer Era[END_REF] confirmed no polycyclic aromatic hydrocarbons (PAH) emission in low-metallicity galaxies, as well as other works (e.g. [START_REF] Wu | Mid-Infrared Properties of Low-Metallicity Blue Compact Dwarf Galaxies from the Spitzer Infrared Spectrograph[END_REF][START_REF] Rosenberg | Dust Properties and Star Formation Rates in Star-Forming Dwarf Galaxies[END_REF]. The HI-to-dust mass ratio as studied by [START_REF] Engelbracht | Metallicity Effects on Dust Properties in Starbursting Galaxies[END_REF] is shown to increase with the decreasing metallicity to 12+log(O/H)s8, as sZ ≠2.5 , and flattens out at lower oxygen abundances. They also found an anti-correlation between dust temperature 3 and metallicity up to 12+log(O/H)s8, from Ts23 K near the Solar metallicity to Ts40 K at lower metallicity, but a positive relation at lower metallicities.

The Herschel Space Observatory further extends the wavelength coverage into the far-IR and submillimeter wavelengths, which combined with the Spitzer photometry significantly improves the measurement of the dust properties. The total gas-to-dust ratio as revealed by Herschel increases the decreasing metallicity. The slope of the relationship is -1 at higher metallicity (12+log(O/H) > 8) and becomes even steeper at the lower metallicity end [START_REF] Rémy-Ruyer | Gas-to-dust mass ratios in local galaxies over a 2 dex metallicity range[END_REF]. It is also found that the metal content may not be the only factor affecting the dust properties of dwarfs. For example, the dust to stellar mass ratio of IZw 18 and SBS 0335-052, which have similar metallicities to the other EMP galaxies in Table 6.1, are very different (10 ≠6 to 10 ≠5 , and 10 ≠3 to 10 ≠2 , respectively) as estimated by [START_REF] Fisher | The rarity of dust in metal-poor galaxies[END_REF] and [START_REF] Hunt | ALMA observations of cool dust in a low-metallicity starburst, SBS 0335-052[END_REF], respectively. The ratio for IZw 18 is extremely low, while that for SBS 0335-052 is comparable to that found for normal Spiral galaxies [START_REF] Hunt | ALMA observations of cool dust in a low-metallicity starburst, SBS 0335-052[END_REF].

While significant progresses have been made in understanding the integrated dust properties of EMP galaxies, spatially resolved investigations of dust properties of these galaxies are still rare, and thus the dependence of dust properties on the local condition is largely unconstrained in the EMP galaxies.

In this study, we aim to benefit from the high spatial resolution of the Herschel Space Observatory to investigate the IR emission of individual star-forming regions of four EMP galaxies, with focus on the IR SED shapes and their relations to the local conditions.

6.2 Sample, observations and data analysis

The sample

Our sample of EMP galaxies contains four objects including Sextans A, ESO 146-G14, DDO 68 and Holmberg II (Ho II). Sextans A is a dwarf irregular at 1.4 Mpc with 12+log(O/H)=7.49 based on the direct method [START_REF] Kniazev | Spectrophotometry of Sextans A and B: Chemical Abundances of H II Regions and Planetary Nebulae[END_REF]. ESO 146-G14 with a direct method based 12+log(O/H)=7.61 [START_REF] Bergvall | ESO 146-G14, a retarded disc galaxy[END_REF] is a blue low surface brightness galaxy at a distance of 21.4 Mpc. DDO 68 with a direct method based 12+log(O/H)=7.21 [START_REF] Pustilnik | Study of DDO 68: nearest candidate for a young galaxy?[END_REF]) is a blue compact dwarf at a distance of 5.9 Mpc. Ho II is a Magellanic irregular dwarf at a distance of 3.3 Mpc (McCall et al., 2012), with a direct method based 12+log(O/H)=7.92 [START_REF] Croxall | Chemical Abundances of Seven Irregular and Three Tidal Dwarf Galaxies in the M81 Group[END_REF].

We also compared our results to the integrated measurements of the KINGFISH sample [START_REF] Kennicutt | KINGFISH-Key Insights on Nearby Galaxies: A Far-Infrared Survey with Herschel: Survey Description and Image Atlas[END_REF], the DGS (Dwarf Galaxy Survey) sample [START_REF] Madden | An Overview of the Dwarf Galaxy Survey[END_REF], and other two EMP galaxies (I Zw 18 & SBS 0335-052). These properties are listed in Table 6.1. [START_REF] Poglitsch | The Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory[END_REF], and 250, 350 and 500 µm through the small map modes with SPIRE [START_REF] Griffin | The Herschel-SPIRE instrument and its in-flight performance[END_REF]. The exposure time (excluding overheads) in the two PACS bands is 1.9 hr, 1.6 hr and 2.7 hr for Sextans A, ESO 146-G14 and DDO 68, respectively, and in the three SPIRE bands is 6 min, 6 min and 10 min, respectively. The data of Sextans A and ESO 146-G14 have been partly published in [START_REF] Shi | Inefficient star formation in extremely metal poor galaxies[END_REF] where the detailed data reduction was given, while the data of DDO 68 is newly presented here. DDO 68 has low S/N, which may cause large uncertainties on the color ( § 6.3). The PACS and SPIRE integration times of the observations are also listed in in Table 6.2. The Herschel data of Ho II was taken from the KINGFISH project [START_REF] Kennicutt | KINGFISH-Key Insights on Nearby Galaxies: A Far-Infrared Survey with Herschel: Survey Description and Image Atlas[END_REF].

In addition to the Herschel data, the Spitzer IR data at 24 µm and GALEX far-UV data at 1516Å were also retrieved from the archive.

Photometric measurements

As detailed in [START_REF] Shi | Inefficient star formation in extremely metal poor galaxies[END_REF], the star-forming disk of each galaxy is defined as an ellipse/circle to closely follow the 10 ‡ contour of the far-UV emission, as shown in Fig. 6.1. Note that for DDO 68 we excluded the tidal tail to better focus on the main disk, although the tail is formally above the 10 ‡ detection threshold. Individual dusty star-forming clumps within a star-forming disk were identified as circle regions with elevated far-UV and 160 µm IR emission which is 3 ‡ above the fluctuations of the disk emission. The defined star-forming regions are listed in Table 6.3. For the flux measurements, the underlying sky emission is estimated within a sky annuli between 1.1 and 1.5 times the disk aperture.

For flux error estimates, the Herschel flux uncertainty is given by the following sources: the first is the uncertainty of the photon noise within the aperture we defined; the second is the uncertainty from the sky background subtraction; the third represents the uncertainty introduced by the offset from the accurate PSF position when we defined the aperture for each source; and the system uncertainty is also included. For more details please see [START_REF] Shi | Inefficient star formation in extremely metal poor galaxies[END_REF]. The measurements of fluxes at other wavelengths basically follow the same procedure. The photometric results of these star-forming regions are listed in Table 6.3.

In order to compare images among different resolutions, aperture corrections are performed on all the star forming clumps based on their PSFs. We have tested that the aperture correction method has almost the same effect as convolving images to the same resolution when fitting SEDs (e.g., [mJy]

[mJy]

[mJy]

[mJy]

[mJy]

[mJy]

[mJy]

SextansA/sf-1 10:10:56.9 see [START_REF] Casasola | The resolved starformation relation in nearby active galactic nuclei[END_REF]. Pixel sizes and resolutions of images at each wavelength are listed in Table 6.4.

6.3

The far-IR SEDs 6.3.1 The color-color diagrams Fig. 6.2-Left shows the f 70µm /f 160µm vs f 160µm /f 250µm color-color diagram of our individual EMP regions along with those integrated measurements of the KINGFISH and DGS galaxies. The predictions of single modified black-body (MBB) models are overlaid as grids where the solid line represents a constant emissivity index, -, with the temperature ranging from 10 to 100 K in decrements of 5 K from the left-bottom to the upper-right, and the dashed lines represent constant temperatures with ranging from 0.0 to 3.0 in steps of 0.5 from the left-bottom to the upper-right. The included DGS galaxies are those with Z/Z above ≥10%, because those EMP ones in their sample lack the enough S/N (Ø 3) to be included. As indicated by Fig. 6.2-Left, our sample shows a quite different behavior as compared to the DGS and KINGFISH Spirals. The majority of the KINGFISH locates in the region with f 70µm /f 160µm < 1.5 and f 160µm /f 250µm < 2.5, and the DGS ranges from the bottom-left to the upper-right, confined by the overlaid gray-body curves. In contrast, our EMP regions mainly occupy the left side of the plot, with f 160µm /f 250µm < 2 but f 70µm /f 160µm covers a range from 0 to 2.5. At given f 160µm /f 250µm our sample shows a much larger scatter in f 70µm /f 160µm , and more dramatically, a large fraction of our sample has f 70µm /f 160µm above the limit of a single modified black-body with -=0 (that is the black-body emission) for a given f 160µm /f 250µm . When compared with the trend of star forming galaxies predicted by empirical models of [START_REF] Dale | A Two-parameter Model for the Infrared/Submillimeter/Radio Spectral Energy Distributions of Galaxies and Active Galactic Nuclei[END_REF], the median behavior of KINGFISH and DGS basically follow the trend, while star-forming regions of our EMP galaxies lie systematically above the trend. A reasonable explanation for the large f 70µm /f 160µm ratio is an excess of f 70µm emission contributed by an additional source of heating. The stochastically heated small grains may be responsible for the excess 70 µm emission [START_REF] Draine | Infrared Emission from Interstellar Dust. IV. The Silicate-Graphite-PAH Model in the Post-Spitzer Era[END_REF]. The requirement of a second heating component when performing a fit to the 70-500 µm photometry has been widely seen in all types of galaxies [START_REF] Galametz | Mapping the cold dust temperatures and masses of nearby KINGFISH galaxies with Herschel[END_REF][START_REF] Rémy-Ruyer | Revealing the cold dust in low-metallicity environments. I. Photometry analysis of the Dwarf Galaxy Survey with Herschel[END_REF], while our sample seems to be the extreme case in which about half of our sample has f 70µm /f 160µm higher than the black-body curve for a given f 160µm /f 250µm color. but the latter shows larger f 160µm /f 250µm at given f 250µm /f 350µm compared to the former. In contrast, our EMP regions mainly occupy the locus toward the bottom-left corner with smaller f 160µm /f 250µm and f 250µm /f 350µm . In the figure there is one object (UM 461) that deviates significantly from the trend.

Similar to

It is difficult to explain such a strange SED. We double checked the photometric measurements and obtained consistent values with those in the literature of [START_REF] Rémy-Ruyer | Revealing the cold dust in low-metallicity environments. I. Photometry analysis of the Dwarf Galaxy Survey with Herschel[END_REF] as used in the figure. But we did notice that the centroids of the 250 and 350 um images are significantly offset (14 ÕÕ ) from those of the 70 and 160 µm, indicating possible significant contamination by a background source at these two wavelengths.

Modified black-body fitting

To further characterize the SED shape and understand the dust emission of EMP star-forming regions, the MBB fittings are carried out. A single MBB model fitting to the full SED is not appropriate as a single photon heating dust component may contaminate significantly the 70 µm or even the 100 µm emission. We first performed a single MBB fitting to the photometry Ø 100 µm by assuming at these longer wavelengths the contribution from the hot dust component was negligible. The MBB model is determined by the dust temperature T and the dust emissivity indexfrom S ‹ = A ú B ‹ (T )⁄ ≠-, where S ‹ was the flux density at frequency ‹, A was a constant that is related to the column density of the dust, and B ‹ (T ) was the Planck function. This method assumed an optically thin condition at the observed far-IR wavelengths with the dust mass derived from M dust = S‹ D 2 Ÿ‹ B‹ (T) , where Ÿ ‹ was the dust opacity, and D was the distance to the galaxy. We took Ÿ ‹ = 1.9 cm 2 g ≠1 at ⁄ = 350µm7 , and calculated the dust mass at ⁄=350µm as this band was less sensitive to temperature than shorter wavelengths, and had lower uncertainties than those at longer wavelengths. As listed in Table 6.5, the single MBB fitting gave a -≥ 1 and T ≥ 20-50 K. The temperature may be underestimated for Sextans A and ESO 146-G14 as there is no 100 µm photometry.

The two-MBB fitting is more reasonable by taking the advantage of the full SED, which is carried out with the equation

S ‹ = A w ú B ‹ (T w )⁄ ≠-w + A c ú B ‹ (T c )⁄ ≠-c
, where A w , T w andw describe the hot component of the dust emission while A c , T c andc are for the cold component. Here we fixedw = 1 to represent the hot small grains andc = 2 for the cold large grains, similar to the study of [START_REF] Zhu | Tracing Molecular Gas Mass in Extreme Extragalactic Environments: An Observational Study[END_REF]. The results are shown in Fig. 6.3 and listed in Table 6.6. The dust mass is derived in the same way as the single MBB fitting. If the cold component dust indicesc decrease from 2.0 to 1.7, 1.5 and 1.3, the derived T c increases and the derived dust mass drops by 10%, 25% and 50%, respectively.

Our two MBB fits give cold dust with T c ≥ 15-20 K and warm dust with T w ≥ 50-60 K. Fig. 6.3 indicates that the warm dust emission is significant for these EMP star-forming regions, in contrast to those of Spirals [START_REF] Galametz | Mapping the cold dust temperatures and masses of nearby KINGFISH galaxies with Herschel[END_REF]. To further characterize the importance of the hot dust emission, we performed similar fittings to the integrated SEDs of the DGS sample [START_REF] Rémy-Ruyer | Revealing the cold dust in low-metallicity environments. I. Photometry analysis of the Dwarf Galaxy Survey with Herschel[END_REF] and investigated the fraction of the emission from the warm dust MBB at 100 µm (f warm /f total ) 100µm as a function of the oxygen abundance. The result is shown in Figure 6.4. It indicates that the fraction (f warm /f total ) 100µm increases on average with decreasing metallicities: around the Solar abundance, the warm dust emission is small (< 20%) but reaches above 50% below one tenth of the Solar metallicity. Such a high warm dust contribution cautions the single MBB fitting to the photometry for EMP galaxies when the data at ⁄ AE 100 µm are included. As shown in Figure 6.3, our two-MBB fittings with -=2 for the cold component also provide good fittings, while the single MBB requires a lower -. This suggests that the result aboutis sensitive to the way how the MBB fitting is performed, and theof metal poor galaxies can be underestimated as a result of a large warm dust contribution up to 100 µm.

Spatial variations of SEDs and dust heating Mechanism

Spatially resolved IR data allow to investigate the spatial variation of the IR SEDs among different star-forming regions in our EMP galaxy sample. As shown in Fig. 6.2-Left, the variation in the color f 70µm /f 160µm among different EMP regions for a given galaxy seems to be smaller as compared to the overall scatter of the integrated color. The f 70µm /f 160µm spans an overall range from about 0.2 to 2.5.

In contrast, four EMP regions in Sextans A have f 70µm /f 160µm between 0.5 and 1.0, four regions of Ho II span the color range from 0.7 to 2.3, and three regions of ESO 146-G14 show the color from 0.2 to 0.7. As shown in Fig. 6.2-Right, the variation in the color f 160µm /f 250µm among different EMP regions within a given galaxy is also smaller than the overall scatter among integrated quantities of galaxies.

With the spatially resolved images, we can further study the color as a function of local conditions in the 2-D. In Fig. 6.5, the far-IR color of individual EMP regions is investigated as a functions of far-UV surface brightness, 24 µm surface brightness, SFRs and stellar masses 9 . All surface brightness/densities as listed in Table . 6.7 are inclination corrected with angles of 0 ¶ , 50 ¶ , 54 ¶ and 30 ¶ for Sextans A, ESO 146-G14, DDO 68 and Ho II, respectively. As indicated by the figure, the far-UV surface brightness of our EMP regions are between about 0.01 and 0.1 MJy/sr, which is within the range of Spirals (Gil de [START_REF] Gil De Paz | The GALEX Ultraviolet Atlas of Nearby Galaxies[END_REF][START_REF] Shi | Extended Schmidt Law: Role of Existing Stars in Current Star Formation[END_REF]. The SFR surface densities of our EMP regions are between 10 ≠3.5 and 10 ≠2 M § /yr/kpc 2 . This is also within the range of star-forming regions of local Spirals [START_REF] Bigiel | The Star Formation Law in Nearby Galaxies on Sub-Kpc Scales[END_REF]. The stellar mass surface densities of EMP regions are between 5 and 15 M § /pc 2 , which is several times lower than those of Spirals (e.g. [START_REF] Shi | Extended Schmidt Law: Role of Existing Stars in Current Star Formation[END_REF]. Fig. 6.5 shows that all three IR colors including f 70µm /f 160µm , f 160µm /f 250µm and f 250µm /f 350µm increase on average with the increasing far-UV surface brightness, 24µm surface brightness and SFR surface densities, with the best relationships with the SFR surface densities. While all these three surface brightness/densities are associated with young stars, the SFR derived here includes both the unobscured component as traced by the far-UV and the obscured one as traced by the 24 µm. Fig. 6.5 further shows that there are no trends between the IR colors and the stellar mass surface densities. Unlike the SFR, the stellar mass is better related to old stars. Investigations of the relationships between the far-IR colors and the tracers can be used to constrain if the dust is mainly heated by the radiation from the young stars or the interstellar radiation field from the old stars. As argued by [START_REF] Bendo | The identification of dust heating mechanisms in nearby galaxies using Herschel 160/250 and 250/350 µm surface brightness ratios[END_REF], such investigations show many advantages in understanding the dust heating mechanism as compared to other methods such as the dust SED fitting and radiative transfer that relies on assumptions of dust grain properties, SED shapes of heating source etc. The results of Fig. 6.5 thus support that young stars are the main heating source of dust radiating at wavelengths from 70 µm to Ø 250 µm in EMP star-forming regions. Investigations of the heating source of dust in Spirals by 9 SFRs and stellar masses are estimated following [START_REF] Leroy | The Star Formation Efficiency in Nearby Galaxies: Measuring Where Gas Forms Stars Effectively[END_REF]'s formula. SFR from [START_REF] Leroy | The Star Formation Efficiency in Nearby Galaxies: Measuring Where Gas Forms Stars Effectively[END_REF] is calibrated from Spiral galaxies and combines both UV (unobsured) and IR (obscured) maps, which is applicable to low-metallicity galaxies [START_REF] Calzetti | The Calibration of Mid-Infrared Star Formation Rate Indicators[END_REF][START_REF] Salim | UV Star Formation Rates in the Local Universe[END_REF]. For one galaxy (Sextans A), we collected the broad-band photometry (FUV, NUV, V-band, 3.6 µm and 4.5 µm) and derived stellar masses based on the SED fitting. The result is not that different from the one based on [START_REF] Leroy | The Star Formation Efficiency in Nearby Galaxies: Measuring Where Gas Forms Stars Effectively[END_REF]'s method, with the difference < 20%.

92

Chapter 6 Spatially resolved dust emission of extremely metal poor galaxies 10 The IR colors as functions far-UV surface brightness, 24 µm surface brightness, SFR surface densities and stellar mass surface densities from left to right.

6.3

The far-IR SEDs [START_REF] Bendo | The identification of dust heating mechanisms in nearby galaxies using Herschel 160/250 and 250/350 µm surface brightness ratios[END_REF] found that in only 3 out of 24 galaxies, the f 160µm /f 250µm and f 250µm /f 350µm are better related to the SFRs than old stars while in the remaining the f 250µm /f 350µm color is driven by both.

The above result suggests that the dust of emission at 70 µm up to 350 µm in EMP regions is heated by young stars instead of diffuse stellar radiation from old stars. The underlying cause for this needs further investigations. As discussed above, the SFR surface densities of our EMP regions are not that different from those in Spirals, indicating the radiation fields from young stars are not enhanced in EMP regions compared to star-forming regions in Spirals. But the SFR relative to the stellar mass, i.e. the specific SFR (sSFR), in these EMP regions is enhanced, with the median of log(sSFR[Gyr ≠1 ]) around -0.82 and a standard deviation of 0.36. In contrast, the star-forming regions of 12 Spirals in [START_REF] Shi | Extended Schmidt Law: Role of Existing Stars in Current Star Formation[END_REF] have the median log(sSFR[Gyr ≠1 ]) of -1.28 and a standard deviation of 0.32. Compared to the sSFRs of dwarf galaxies as studied by Hunt et al. (2015b), our galaxies lie below their trend, but still within the scatter (0.01 -30 Gyr ≠1 , around metallicities of our galaxies). This difference could be due to the fact that our galaxies are mostly dwarf irregulars and low-surface-brightness galaxies, while the sample of Hunt et al. (2015b) contains many blue compact dwarf galaxies that are known to be compact with enhanced SFRs and have higher sSFRs due to interactions.

The properties of dust grains in EMP galaxies may be systematically different from those in Spirals, making the heating from young stars important to the dust emission all the way up to 350 µm. For example, if small dust grains in EMP galaxies are abundant, they could be heated to higher temperatures without requiring an enhanced radiation field. It is observationally difficult to quantify the size of dust grains. Studies of extinction curves point out that metal poor dwarfs in the local group, SMC and LMC, show more steeply rising extinction at UV wavelengths, suggesting smaller dust grains in these two galaxies (for a review, see [START_REF] Li | Dust in the Local Group[END_REF]. A simple extrapolation of this result into the EMP regime would naturally expect the dust in EMP galaxies to be smaller due to the lack of raw material for grain growth. Small grains could offer more surface area for the formation of molecular hydrogen that is likely abundant in EMP galaxies as indicated by several tracers including dust (e.g. [START_REF] Shi | Inefficient star formation in extremely metal poor galaxies[END_REF], warm H 2 (e.g. [START_REF] Hunt | The Spitzer View of Low-Metallicity Star Formation. III. Fine-Structure Lines, Aromatic Features, and Molecules[END_REF] and [CII] 158 µm (e.g. [START_REF] Madden | C II] 158 Micron Observations of IC 10: Evidence for Hidden Molecular Hydrogen in Irregular Galaxies[END_REF][START_REF] Madden | An Overview of the Dwarf Galaxy Survey[END_REF] although CO is very weak (e.g. Shi et al., 2015b). However, it is impossible with current facilities to obtain the measurements of the extinction curve for these relatively distant EMP galaxies, and thus to conclude the size of dust grains in EMP galaxies.

Dust-to-stellar mass ratio

In Fig. 6.6, the dust-to-stellar mass ratio of our EMP regions are plotted against the oxygen abundance, 12+log(O/H), along with the integrated quantities from the DGS. The dust masses of both samples are measured using the same method, i.e. the two MBB fitting (see §6. The dust-to-stellar mass ratio of our EMP star-forming regions and integrated galaxies of the DGS.

The lines are the model predictions by [START_REF] Feldmann | The equilibrium view on dust and metals in galaxies: Galactic outflows drive low dust-to-metal ratios in dwarf galaxies[END_REF]. tISM is the dust growth time-scale which can be computed from basic collision theory [START_REF] Weingartner | Interstellar Depletion onto Very Small Dust Grains[END_REF] (solid or dashed lines). QMS denotes the (multiplicative) offset of a given galaxy from the main sequence (blue: 1/3, cyan: 1, green: 3). The data for IZw 18 and SBS 0335-052 are from [START_REF] Hunt | ALMA observations of cool dust in a low-metallicity starburst, SBS 0335-052[END_REF].

mass ratio is not related to the metallicity. The figure further indicates that even within a given galaxy, the dust-to-stellar mass ratio of individual EMP regions could show large scatters, e.g. EMP regions in Ho II span the whole range from roughly 10 ≠4 to 10 ≠2 while EMP regions in Sextans A and ESO 146-G14 have much smaller scatters.

The overlaid lines in Fig. 6.6 are the predictions of the model by Feldmann (2015, private communication), where t ISM is the dust growth time-scale which can be derived from Weingartner and Draine (1999)'s basic collision theory (solid or dashed lines). Blue, cyan and green lines refer to Q MS = 1 3 , 1, 3, which denote the (multiplicative) offset of a given galaxy from the main sequence. While the normalization of the trend depends on the methodology used for dust mass and metallicity measurements, the model-predicted sharp drop between 1/10 and 1/5 solar abundance is clearly not seen in Fig. 6.6. The model of [START_REF] Feldmann | The equilibrium view on dust and metals in galaxies: Galactic outflows drive low dust-to-metal ratios in dwarf galaxies[END_REF] invokes gas outflow, inflow and star formation in an equilibrium state in order to reproduce the observed dust-to-gas mass ratio as a function of metallicity as well as many other observed galaxy properties. The model is motivated to explain the observed sharp drop in the dust-to-gas mass ratio vs. the metallicity around 20% solar metallicity [START_REF] Rémy-Ruyer | Gas-to-dust mass ratios in local galaxies over a 2 dex metallicity range[END_REF][START_REF] Shi | Inefficient star formation in extremely metal poor galaxies[END_REF]. Fig. 6.6 indicates that although a sharp drop may occur in the dust-to-gas ratio, a similar drop in the trend of the dust-to-stellar ratio as a function of metallicity is not seen. The model argues that the dust content in EMP galaxies is mainly regulated by the galactic outflow, whose efficiency may be overestimated so that too much dust is removed relative to the stellar content. Our results further suggest that if outflows regulate the dust-to-stellar mass ratio, they must vary greatly on scales of 100 -1000 pc.

Conclusion

In this thesis, I focus on the large-scale (≥ kpc) star formation of galaxies, taking advantage of the data from Herschel, ALMA, NOEMA, and the SAMI IFU survey.

Based on the GOODS-ALMA survey, the largest cosmological survey with the large ALMA interferometer at 1.1 mm, I studied the six optically-dark galaxies (i.e., undetected in the observed U to H-bands) uncovered by this survey. We present evidence that four out of the six optically-dark galaxies belong to the same overdensity of galaxies at z ≥ 3.5. One of them, AGS24 (M ı = 10 11.32 +0.02 ≠0.19 M § ), is the most massive galaxy without an AGN at z > 3 in the GOODS-ALMA field, and it falls in the very center of the galaxy surface density peak. This suggests that the surrounding overdensity is a proto-cluster in the process of virialization and that AGS24 is the candidate progenitor of the future brightest cluster galaxy. This further indicates that wide but moderately shallow surveys at submilimeter wavelengths are a powerful tool to unveil massive galaxies which tend to reside at the bottom of the potential well of large-scale structures, leading us to follow up the properties of such galaxies in the context of their environments.

The nearby extremely metal-poor galaxies can be considered as analogs of the high-z galaxies which also form stars out of gas with very low metal content. They serve as ideal laboratories to study the formation and evolution of galaxies in the early Universe. I studied the molecular gas content in IZw18, one of the most metal poor galaxies. The upper limit of CO J=2-1 emission is ten times lower than previous studies despite the vigorous star formation activity of IZw18. Such low CO content indicates a drastic change in the structure of the ISM at around a few percentages of Solar metallicity. Especially, the high [C II] luminosity relative to CO implies a larger molecular gas reservoir than inferred from CO in IZw18. I also studied the resolved dust emission in four extremely metal-poor galaxies observed by Herschel. They show higher dust temperatures and lower emissivity indices compared to those of spiral galaxies. The high contribution from warm (50 K) dust to the far-IR emission and the correlation between the far-infrared colours and the surface densities of young stars suggest that the dust emission of these galaxies is primarily heated by radiation from young stars.

The SAMI IFU survey provides 3D information on galaxy kinematics. My work shows that on sub-kpc scales, local star-forming galaxies display a flat distribution of ionized gas velocity dispersion as a function of star formation rate surface density. However, the velocity dispersion is higher than the prediction of star-formation-driven models, implying additional sources to drive random motions of the interstellar medium in star-forming galaxies.

In summary, the work done in my thesis has put more constraints on the understanding of galaxy evolution: The first galaxies with little heavy elements may have very different ISM structures, very likely to be more porous, compared the local galaxies. And the study on the gas content in such galaxies requires tracers such as [C II] [START_REF] Madden | Tracing the total molecular gas in galaxies: [CII] and the CO-dark gas[END_REF] other than CO, which can be easily destroyed without the shielding by dust. The radiation field is more exposed to the ISM due to the low dust content. Then it's harder to increase the density of molecular clouds. This may lead to the low star formation efficiency in these galaxies [START_REF] Shi | Inefficient star formation in extremely metal poor galaxies[END_REF]. As galaxies gradually grow massive, their high masses make it easier to sustain the dust content hence they form stars more efficiently. Galaxies reside in dense environments tend to experience an accelerated growth and end up to be more massive than those in isolated systems, as they may have had more abundant gas reservoir to form stars and/or more interactions with other galaxies. This could be the case of the massive galaxies we found in the protocluster at z ≥ 3.5, which are rare and extremely faint in the optical to near-IR due to the high dust content. However, an early mature may indicate an early death since the rapid growth can exhaust the fuel of star formation very shortly, which is the case of the massive quiescent galaxies observed at z ≥ 2. The establishment of this evolution path requires more statistics. The star-forming galaxies are in the balance between the gravity and pressure. The energy sources to support the pressure in distant and local galaxies are not the same, with the former from the star formation feedback and the later from the gravitational energy release of gas radial transportation. The transition between the two can be also related to the morphological transformation where the bulges form early in compact distant galaxies and discs seen in the local galaxies form later [START_REF] Krumholz | A unified model for galactic discs: star formation, turbulence driving, and mass transport[END_REF].

7.2 Perspective 7.2.1 More on the optically dark galaxies Spectroscopic redshifts and the ISM properties of the optically dark galaxies in the protocluster: The redshift measurements in Chapter 3 are based on the analysis of optical-to-near-IR SED, far-IR SED, as well as the one emission line detected in two of the optically-dark galaxies. It is necessary to obtain the precise redshifts spectroscopically by observing the bright [C II] line, which is considered as the main coolant in high-z galaxies, emitting ≥0.1%-1% of the far-IR continuum. The [C II]/L FIR ratio shows an anticorrelation with L FIR , but with a large scatter. Meanwhile, recent studies argue that [C II]/L FIR exhibits a tighter relation with dust temperature (e.g., [START_REF] Malhotra | Herschel Extreme Lensing Line Observations: [CII] Variations in Galaxies at Redshifts z=1-3[END_REF]. The high frequency of the [C II] line also enables a wider redshift range search in comparison with the CO lines, given the limited observing frequency range of ALMA. A follow-up observation of [C II] with ALMA would allow us to confirm the redshift of these galaxies and study the properties of the ISM in these galaxies. Once the redshifts are confirmed, we would request to detect their CO emission by VLA or ALMA. [START_REF] Ginolfi | Molecular gas on large circumgalactic scales at z = 3.47[END_REF] already showed evidence of a CO gas stream surrounding one of the optically-detected galaxies in this structure. We expect to see the molecular gas not only in the optically dark galaxies fueling the star formation but also across the entire large-scale structure supporting the continuous growth of the galaxies therein.

Environments of a larger sample of optically dark galaxies:

The overdensities of dusty starforming galaxies (DSFGs) haven been discovered since ALMA revealed the multiplicities of the brightest Herschel and JCMT/SCUBA2 sources. These dusty galaxies always show extremely active star formation. The existence of DSFGs challenges current models, and whether proto-clusters traced by these galaxies are rare or common is still under debate. Our team has identified a large sample of optically dark galaxies in three CANDELS fields based on their H -[4.5] color [START_REF] Wang | Infrared Color Selection of Massive Galaxies at z 3[END_REF], see also Section 1.2.1. These optically dark galaxies generally fall in the galaxy main sequence, in contrast to those bright DSFGs. Owing to the abundant ancillary data in the CANDELS fields, we can construct galaxy number densities for these galaxies to see the environments they reside in as we did in [START_REF] Zhou | GOODS-ALMA: Optically dark ALMA galaxies shed light on a cluster in formation at z = 3.5[END_REF]. The difference shown by the proto-clusters traced by the bright DSFGs and by the main sequence optically-dark galaxies would constrain how dense environments affect the evolution of the galaxies therein.

In the era of the JWST: accurate measurements of stellar properties of the optically dark galaxies: Because of the extreme faintness of the optically dark galaxies, the stellar mass measurements have been limited to the photometries from the marginal detections or blended images, let alone the size measurements of their stellar component. With the high sensitivity and high angular resolution of the near-to-mid IR instruments on the upcoming JWST telescope, it is possible to obtain images with high enough quality to derive the stellar properties of these galaxies.

AGN feedback on high-z star-forming galaxies

As mentioned in 2.1.3, with the newly arrived GOODS-ALMA data (Cycle 5), I stacked the star-forming galaxies with and without AGNs using the merged GOODS-ALMA image (Cycle 3 and Cycle 5). I found that AGNs galaxies have up to 50% higher flux densities at 1.1 mm, indicating that AGNs do not decrease the SFRs hence do not quench these star-forming galaxies. This is not contradictary to the high AGN fraction found in the GOODS-ALMA detections [START_REF] Franco | GOODS-ALMA: 1.1 mm galaxy survey. I. Source catalog and optically dark galaxies[END_REF] and is comparable to the excess of AGNs among starbursts at z = 2 in [START_REF] Rodighiero | Active Galactic Nuclei in Dusty Starbursts at z = 2: Feedback Still to Kick in[END_REF]. I will keep working on the influence of AGNs on high-z star-forming galaxies.

Acknowledgement

It came as a surprise when I got to know that I could also do a joint-PhD at Université de Paris Saclay and it was already such a great pleasure to come to CEA as a visiting student for two years, thanks to everyone at CEA as a host and the financial support from China Scholar Council.

During the two years in France, I have gained a lot of knowledge not only from academia but also from everyone I met.

First of all, thank you, David. Thanks for hosting me at CEA and for mentoring me in this thesis. Your passion for science inspires me. It reminds me that being a researcher is not only a way to make a living, but more essentially a lifestyle. A lifestyle to stay curious about everything, to observe what is happening around us, to crack down what is behind what we see. It reminds me of how I was originally fascinated about science: to think, to ask questions originated from my daily life and finally to describe, to explain, and to predict these phenomena. I am grateful for your patience with me, for explaining to me the big pictures and for so much help with my life in France.

Thanks Emanuele for always taking so much time for the Journal Club and for the valuable discussions, your door is always open when I need help. Thanks also to many other people, Jérôme, Alain, Pascale, Laure, Tao, Corentin, Benjamin, Frederic, Emeric, Koryo for always being so warm-hearted, I have learnt a lot from you. And all the friends at CEA, my two years at CEA would not have been so fruitful and unforgettable without you. Max, my office mate, you have been always helping me in the most critical time. You offered me valuable suggestions when I got stuck at work. And our tea/coffee breaks, bicycle trips, cheese and wine... Chiara, I feel so lucky to have had you by my side during all these adventures (the opening day, the D-day, the interviews with the committees... and our trip to Giverny, hanging out in Paris). Antonello, you are one of the old friends since I came to CEA for a short internship three years ago. You have been always sharing your good ideas with me. I am so grateful for your immediate help after my laptop was stolen. Anna, you always have the magic to make the atmosphere so lively and cheerful. Ivan, you are so organized and taking care of everyone of us. Baptise and Solène, you introduced to me the French culture and shared with me so many fun facts about life in France. Dangning, thanks to you, our "2019 Chinese New Year's Eve dinner" was such a success. You also offered me so many useful tips as an experienced "Chinese in France". And many other friends I met at CEA: Rose, Maud, Valeska, Benjamin, Antigone, Anita, Daizhong, Francesco, Jeremy.

And my life at the center of the world, Gif-sur-Yvette: Thank you, Judica, for hosting me at your place. You have been like a mother to teach me how to take care of myself in your own special way, and have shown me how to think critically, how to take actions actively to solve problems efficiently.

Thank you, Pascale. You have never regarded me as an outsider, and have always kindly invited me to all kinds of activities. This makes me feel integrated to the life in France and so much less lonely when I was in France.

3.4

Images of AGS4 from the observed B (435 nm, rest-frame UV) to near-infrared observed K s (2.2 µm, rest-frame B) bands. The red contours denote the ALMA detection at 1.1mm at the resolution of 029. The green dashed contours denote the ALMA detection at 2mm at the resolution of 088. The astrometric corrections between ALMA and HST images are applied throughout this work. The cyan "x" denotes the optical bright neighbor, ID CANDELS = 8923 (z phot = 0.24). The magenta "x" denotes the detection by ZFOURGE based on the K s -band image, ID ZFOURGE = 12333 (z phot = 3.76). If not stated otherwise, the images in this paper are all oriented with the North up and East to the left. . . . . . .

3.5

Images of AGS25 in the H-band (1.6 µm), K s -band (2.2 µm) and IRAC1-band (3.6 µm).

The contribution of a 3 ÕÕ distant bright neighbor of AGS25 was subtracted to produce the IRAC image as in F20a. The red contours denote the ALMA detection at 1.1mm at the resolution of 060. The magenta "x" is the ZFOURGE counterpart ID ZFOURGE = 11353. . .

3.6

Redshift distribution of the 11674 galaxies with a redshift (either photometric or spectroscopic) located within the 6.9 Õ ◊10 Õ GOODS-ALMA field (grey). The sub-sample of 395 galaxies located within 50 ÕÕ of the most massive galaxy in the field at z > 3, AGS24, are shown in blue. The insert shows the distribution of the spectroscopic redshifts of the 121 galaxies at 3 AE z spec AE 4 over the GOODS-ALMA field. Ten galaxies fall in the redshift bin 3.470 AE z spec AE 3.480. The mean redshift of the ten galaxies is z spec = 3.472. The redshifts are from the ZFOURGE [START_REF] Straatman | The FourStar Galaxy Evolution Survey (ZFOURGE): Ultraviolet to Far-infrared Catalogs, Medium-bandwidth Photometric Redshifts with Improved Accuracy, Stellar Masses, and Confirmation of Quiescent Galaxies to z 3.5[END_REF]

3.7

Images of AGS24 in the B, V, H and K s band, IRAC 3.6 and 4.5µm, JVLA 6 GHz (3.7 ‡) and 3 GHz (5.7 ‡). The contours are the same as in Fig. 3.4 but for AGS24. The three neighbor galaxies are labeled with ID ZFOURGE or ID CDFS and their redshift separately. . . 3) galaxies. The yellow stars refer to the HST-galaxies AGS 11, 15 and 17 located also in this larger structure. The green circles represent the AGS 1 and 5 which are also in this redshift range but not optically-dark. The white circles show the 40 galaxies with spectroscopic redshifts. We note that the two white circles falling on the red star of AGS24 are two galaxies dissociated from AGS24. The magenta square denotes the center of the proto-cluster CCPC-z34-002 identified by [START_REF] Franck | The Candidate Cluster and Protocluster Catalog (CCPC) of Spectroscopically Identified Structures Spanning 2.74 &lt[END_REF].

The cyan square denotes Candels5001, which used to be the most massive galaxy at 3 < z < 4 in this field [START_REF] Ginolfi | Molecular gas on large circumgalactic scales at z = 3.47[END_REF]. The 50 ÕÕ scale in the figure is equivalent to a proper distance of 0.37 Mpc at z = 3.5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.

12 Comparison of the auto-correlation function (ACF) of the 364 galaxies at 3.42 AE z AE 3.57 in the ZFOURGE catalog (red dots), the cross-correlation function (CCF) between these galaxies and the ALMA detections (black triangles), and the cross-correlation function between these galaxies and the galaxies which are equivalently massive as the ALMA detections (green triangles), i.e., M ı > 10 10.5 M § . . . . . . . . . . . . . . . . . . . . . . . .

3.13

Projected numbers of galaxies in the over-density as a function of their distance to AGS24 (black circles) and the black dashed line is the linear fitting to the profile. The red line shows the best-fit projected NFW profile [START_REF] Navarro | The Structure of Cold Dark Matter Halos[END_REF]

4.3

The infrared luminosity and SFR (Top-left) versus L Õ CO(1≠0) (Bottom-left) as well as the SFR/L Õ CO(1≠0) as a function of metallicity. The upper limit of IZw18 is show as a black circle with an arrow, in comparison with the galaxies described in Section 5.4. In the two panels on the left, the blue dashed lines show the linear correlation by [START_REF] Gao | The Star Formation Rate and Dense Molecular Gas in Galaxies[END_REF] and the red dashed lines show the linear fit of the metal poor galaxies (Shi et al., 2015a;[START_REF] Shi | Carbon monoxide in an extremely metal-poor galaxy[END_REF]Hunt et al., 2015a;Cormier et al., 2014, DGS). In the right panel, the blue solid line denote the best fit of the metal poor galaxies in Hunt et al. (2015a) where galaxies in [START_REF] Schruba | Low CO Luminosities in Dwarf Galaxies[END_REF] are also included. The black solid line shows a power law fitting all the galaxies in the plot. .

5.1

Images of different components of each galaxy. Top: SFR surface density (extinction corrected) in M § yr ≠1 kpc ≠2 . Middle: ionised gas velocity in km s ≠1 . Bottom: ionised gas velocity dispersion in km s ≠1 . In all images, we only display spaxels with S/N > 34 (see Section 5.2.2.2 for the choice of S/N). The red crosses are the centres of the galaxies.

The white solid lines are the major axes (see Section 5.2.2.1). . . . . . . . . . . . . . . . .

5.2

Dependence of gas velocity dispersion ( ‡ gas ) on velocity gradient (v grad , Equation 5.1).

Points in each individual galaxy are labeled by colour. The solid line refers to ‡ gas = 2 v grad and the dashed line refers to ‡ gas = v grad . The dotted line denotes the velocity gradient at 20 km s ≠1 . These three different lines represent the three different criteria that we tested to account for beam smearing (Section 5.2.2.3). . . . . . . . . . . . . . . . . . . . . . . .

5.3

Spatially resolved dependence of ‡ gas on SFR . We remove pixels with v grad > 0.5 ‡ gas to account for beam smearing effects (see Figure 5.2 and Section 5.2.2). Our eight SAMI galaxies are compared to Lehnert et al. (2009, Figure 12 therein). Each filled circle refers to one spaxel in each galaxy and is colour-coded with the magnitude of velocity gradient (v grad , Equation 5.1). The crosses and asterisks in different colours refer to the 11 actively star-forming galaxies at z ≥ 2 in [START_REF] Lehnert | Physical Conditions in the Interstellar Medium of Intensely Star-Forming Galaxies at Redshift˜2[END_REF]. The solid black curves show ‡ Ã (' Ė) 1/2 , where Ė is the energy injection due to star formation, and ' is the coupling efficiency of the energy injected into the ISM. The dashed curves show ‡ Ã (' Ė) 1/3 assuming that velocity dispersions correspond to energy dissipation due to turbulent motions. The red solid curve shows the velocity dispersion of a 10 8 M § clump assuming a simple Jeans relation. A zoom-in of our SAMI galaxies with a logarithmic y-axis is also shown here.

5.4

Global dependence of ‡ gas on SFR . Our eight SAMI galaxies compared to local high H-luminosity galaxies from [START_REF] Green | DYNAMO -I. A sample of H--luminous galaxies with resolved kinematics[END_REF] and z > 1 star-forming galaxies and clumps (see Section 5.3.2.2 for further details). Each blue (red) filled diamond shows one entire galaxy in our sample including (excluding) the contribution from thermal broadening ( ‡ thermal ≥ 12 km s ≠1 , [START_REF] Glazebrook | The Dawes Review 1: Kinematic Studies of Star-Forming Galaxies Across Cosmic Time[END_REF]. For the measurement of ‡ gas and SFR , see footnotes in Table 6.1. Green diamonds refer to the H-luminous galaxies in [START_REF] Green | DYNAMO -I. A sample of H--luminous galaxies with resolved kinematics[END_REF]. The black filled squares, dark green triangles, red open squares and blue open circles refer to the z > 1 star-forming galaxies and clumps. The grey contour denotes the distribution of local star-forming galaxies with gas velocity dispersion derived from HI [START_REF] Dib | The Supernova Rate-Velocity Dispersion Relation in the Interstellar Medium[END_REF], and we include the intrinsic thermal broadening of 12 km s ≠1 here. . . . . . Left: The distribution of EMP star-forming regions, and integrated measurements of galaxies from DGS and KINGFISH in the f 70µm /f 160µm vs. f 160µm /f 250µm color-color diagram. The grid is a single modified black-body with a range of temperatures and emissivity indices. The solid line represents a constant emissivity index, -, with the temperature ranging from 10 to 100 K in a decrement of 5 K from the left-bottom to the upper-right. Dashed lines represent constant temperature withranging from 0.0 to 3.0 in a step of 0.5 from the left-bottom to the upper-right. The brown line is the trend of star forming galaxies predicted by the model of [START_REF] Dale | A Two-parameter Model for the Infrared/Submillimeter/Radio Spectral Energy Distributions of Galaxies and Active Galactic Nuclei[END_REF] The fraction of warm dust emission at 100 µm as a function of the Oxygen abundance based on the two modified black-body fitting. The red diamonds are the EMP star-forming regions, and blue circles are for the integrated dwarf galaxies from the DGS. . . . . . . .

6.5

The IR colors as functions far-UV surface brightness, 24 µm surface brightness, SFR surface densities and stellar mass surface densities from left to right. . . . . . . . . . . .

6.6

The dust-to-stellar mass ratio of our EMP star-forming regions and integrated galaxies of the DGS. The lines are the model predictions by [START_REF] Feldmann | The equilibrium view on dust and metals in galaxies: Galactic outflows drive low dust-to-metal ratios in dwarf galaxies[END_REF]. t ISM is the dust growth time-scale which can be computed from basic collision theory [START_REF] Weingartner | Interstellar Depletion onto Very Small Dust Grains[END_REF] (solid or dashed lines). Q MS denotes the (multiplicative) offset of a given galaxy from the main sequence (blue: 1/3, cyan: 1, green: Dans cette th èse, je me concentre sur la formation des étoiles aux grandes échelles des galaxies (⇠kpc). En utilisant les donn ées des t él éscopes Herschel, ALMA, NOEMA, et SAMI IFU, ma th èse tente de mettre en lumi ère les questions suivantes : (1) Comment les premi ères galaxies ont-elles form é des étoiles à partir du gaz contenant peu de m étaux ? (2) Comment les galaxies massives du d ébut de l'Univers ont-elles rapidement constitu é leur masse et finalement cess é de former des étoiles ? (3) Quel est le r ôle jou é par les environnements dans lesquels elles r ésident ? (4) Comment la r étroaction de la formation des étoiles affecte-t-elle la cin ématique des galaxies ? Premi èrement, je pr ésente l' étude de six galaxies optiquement sombres extraites de l' étude GOODS-ALMA (la plus grande étude cosmologique à 1,1m) dont le d écalage vers le rouge est sup érieur à z = 3. Je pr ésente des preuves que quatre d'entre elles appartiennent à la m ême surdensit é de galaxies, situ é à z ⇠ 3.5. L'une d'entre elles, AGS24, est la galaxie la plus massive, sans noyau galactique actif et situ é à z>3. Cette derni ère se situe également au centre du pic de densit é de surface des galaxies, sugg èrant que la surdensit é environnante est un proto-amas en cours de virialisation et qu'AGS24 est un candidat prog éniteur d'un futur amas de galaxie plus brillant. Deuxi èmement, en me basant sur une observation NOEMA, j'ai étudi é la teneur en gaz mol éculaire dans IZw18, l'une des galaxies les plus pauvres en m étal de l'Univers local. J'ai obtenu une limite sup érieure d' émission de CO(J=2-1) qui est dix fois plus faible que pr éc édemment d étermin ée, ceci malgr é la forte activit é de formation d' étoiles. Sa faible teneur en CO (par rapport à sa luminosit é infrarouge), son taux de formation d' étoiles, et sa luminosit é [CII] indiquent un changement drastique dans la structure du MIS (environ quelques pourcents de la m étallicit é solaire). En particulier, la luminosit é élev ée [CII] par rapport au CO implique un r éservoir mol éculaire plus important que l' émetteur de CO dans l'IZw18. Troisi èmement, mes travaux sur les sources d' énergie des mouvements turbulents montrent qu' à des échelles inf érieures au kpc, les galaxies locales formant des étoiles du relev é SAMI pr ésentent une distribution plate de la dispersion de vitesse du gaz ionis é en fonction de la densit é de surface du taux de formation d' étoiles. Cependant, le plancher de dispersion de vitesse est plus élev é que ce que pr édisent les mod èles "feedback-driven". Cela sugg ère que des sources suppl émentaires à la r étroaction de formation d' étoiles entraînent des mouvements al éatoires du MIS dans les galaxies formant des étoiles. Enfin, je pr ésente mes travaux sur l' émission de poussi ères spatiallement r ésolue des galaxies extr êmement pauvres en m étaux observ ées par Her-schelCes galaxies pr ésentent des temp ératures de poussi ère plus élev ées et des indices d' émissivit é plus faibles que ceux des galaxies spirales. Environ la moiti é de l' émission à 100um provient de la poussi ère chaude (50K), par opposition à la composante froide (⇠20K) de la poussi ère. Les couleurs de l'infrarouge lointain sont toutes li ées aux densit és de surface des jeunes étoiles, mais pas aux densit és de surface des masses stellaires. Cela sugg ère que leur poussi ère (70⇠350µm) est principalement chauff ée par le rayonnement des jeunes étoiles.

Universit é Paris-Saclay

Espace Technologique / Immeuble Discovery Route de l'Orme aux Merisiers RD 128 / 91190 Saint-Aubin, France Title : Probing the processes driving distant and local star-formation in galaxies through dust and molecules Keywords : galaxy evolution, star formation, interstellar medium, ALMA, Herschel, NOEMA Abstract : Studies on galaxy evolution have been revolutionized during the last decade thanks to the state-of-the-art telescopes and instruments. Herschel Space Telescope, ALMA and NOEMA interferometers can observe the dust emission that traces star formation missed by optical to near-infrared telescopes. Their high sensitivity pushes further their detections to the most distant galaxies or the faintest local galaxies, of which the formation and evolution challenge current theories and models. And the integral field unit technique combines the spatial information with the properties extracted from spectra which allows a thorough study of galaxy kinematics. In this thesis, I focus on the large-scale (⇠ kpc) star formation of galaxies. Taking advantage of the data from Herschel, ALMA, NOEMA, and the SAMI IFU survey, my thesis tries to put more constraints on the following questions: (1) How did the first galaxies form stars out of gas with little metal? (2) How did the massive galaxies in the early Universe rapidly build up their masses and finally stop forming stars? and ( 3) what is the role played by the environments they reside in? (4) How does star formation feedback affect the galaxy kinematics? A large portion of the work done in this thesis is based on the GOODS-ALMA survey, the largest cosmological survey with the large ALMA interferometer at 1.1 mm. I studied the six optically dark galaxies with redshifts greater than z = 3 uncovered by this survey. We present evidence that four out of the six optically dark galaxies belong to the same overdensity of galaxies at z ⇠ 3.5. One of them, AGS24, is the most massive galaxy without an active galactic nucleus at z > 3 in the GOODS-ALMA field. It also falls in the very center of the galaxy surface density peak, suggesting that the surrounding overdensity is a proto-cluster in the process of virialization and that AGS24 is the can-didate progenitor of the future brightest cluster galaxy. I also studied the molecular gas content in IZw18, one of the most metal poor galaxies in the local Universe based on the observation of NOEMA. I obtain an upper limit of CO J=2-1 emission, which is used as a tracer of molecular gas, to be ten times lower than previous studies despite its vigorous star formation activities. Such low CO content relative to its infrared luminosity, star formation rate and [C II] luminosity, indicates a drastic change in the structure of the ISM at around a few percentages of Solar metallicity. Especially, the high [C II] luminosity relative to CO implies a larger molecular reservoir than the CO emitter in IZw18. My work on the energy sources of the turbulent motions shows that on sub-kpc scales, local star-forming galaxies from the SAMI survey display a flat distribution of ionized gas velocity dispersion as a function of star formation rate surface density. However, the velocity dispersion floor is higher than predicted by feedback-driven models. This suggests that additional sources to star formation feedback drive random motions of the interstellar medium in star-forming galaxies. Finally I present my work on the spatially resolved dust emission of extremely metal-poor galaxies observed by Herschel. These galaxies show higher dust temperatures and lower emissivity indices compared to those of spiral galaxies. And about half of the emission at 100 µm comes from warm (50 K) dust, in contrast to the cold (⇠20 K) dust component. The farinfrared colours are all related to the surface densities of young stars, but not to the stellar mass surface densities. This suggests that their dust emission (70 ⇠ 350 µm) is primarily heated by radiation from young stars.

Universit é Paris-Saclay

Espace Technologique / Immeuble Discovery Route de l'Orme aux Merisiers RD 128 / 91190 Saint-Aubin, France

Fig. 1 . 2 :

 12 Fig. 1.2: The SED of a typical Lyman break galaxy at z ≥ 3, and the images of this galaxy taken with photometric filters at Un, G, Rs bands. Courtesy: CANDELS blog.

Fig. 1 . 3 :

 13 Fig. 1.3: Left: The galaxy absolute magnitude-color diagram in SDSS filter colors, where absolute magnitude has been converted to stellar mass. Figure adapted from Schawinski et al. (2014) by S. Majewski. Right: Schematic representation of the main sequence and other galaxies with different star formation modes. Courtesy: CANDELS blog.

Fig. 1 . 5 :

 15 Fig. 1.5: Cosmic star formation rate density (SFRD). a: Cosmic SFRD before dust attenuation correction. b: Mean dust extinction in magnitude as a function of redshift. c: Cosmic SFRD derived from the UV data after dust attenuation correction and from IR, respectively. d: Cosmic SFRD derived from the combination of the UV data after dust attenuation correction and IR data. Figures from Madau and Dickinson (2014).

1. 2

 2 Star formation obscured by interstellar dust

Fig. 1 . 6 :

 16 Fig. 1.6: Illustration of K-correction: The observed flux densities for a typical 10 12.5 L § infrared-luminous galaxy as a function of redshift. The curves are color coded with the observed wavelength. Figure from Casey et al. (2014).

Fig. 1 . 7 :

 17 Fig. 1.7: Illustration of the SED the Cosmic Microwave Background (CMB), the Cosmic Infrared Background (CIB) and the Cosmic Optical Background (COB). Their approximate brightness in nW m ≠2 sr ≠1 is marked in the boxes. Figure from Dole et al. (2006).

Fig. 1

 1 Fig. 1.8: Left: SFR as a function of stellar mass of the SMGs at 1.7 < z < 2.6 studied by Lang et al. (2019). The solid and dotted lines indicate the main sequence and the 0.3 dex scatter from Whitaker et al. (2014). Right: Histograms of the effective radii for the ALMA 870µm continuum emission, stellar mass profiles, and H-band emission from the same work. Figures from Lang et al. (2019).

Fig. 1 . 9 :

 19 Fig. 1.9: GOODS-ALMA 1.1mm image tapered at 0 ÕÕ 60, reaching an rms of 0.18 mJy beam ≠1 (Franco et al., 2020b). The white and blue circles with a diameter of 4 ÕÕ indicate the blind detections with S/N above the 4.8 ‡ and detections with priors at 3.5 ≥ 4.8 ‡, respectively. Black contours show the different slices (labeled A to F) used to construct the homogeneous 1.1 mm coverage. Blue lines indicate the HST/ACS field and the green lines indicate the HST-WFC3 deep field. The cyan contours represent the limit of the Dunlop et al. (2017) survey covering all of the Hubble Ultra Deep Field region, the yellow contours show the ASAGAO survey[START_REF] Hatsukade | ALMA twenty-six arcmin 2 survey of GOODS-S at one millimeter (ASAGAO): Source catalog and number counts[END_REF], the gray contours show the ASPECS Pilot survey[START_REF] Walter | ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: Survey Description[END_REF], the pink contours show the ASPECS Large Program[START_REF] Decarli | The ALMA Spectroscopic Survey in the HUDF: CO Luminosity Functions and the Molecular Gas Content of Galaxies through Cosmic History[END_REF].

Fig. 1 .

 1 Fig. 1.10: Left: Contribution of optically dark galaxies to the cosmic SFRD. Figure from Wang et al. (2019). Right: Upper panel: SFR density for all galaxies (black), protoclusters (blue), and cores (red). Lower panel: Fractional contributions to the total cosmic SFRD. Figure from Chiang et al. (2017).

Fig. 1 .

 1 Fig. 1.11: Illustration of "CO dark" gas in molecular clouds. Figure from Madden et al. (2020).

Fig. 1 .

 1 Fig. 1.12: Left: Spatially resolved atomic (a), molecular (b) and total (d) gas -star formation law and gas column density distribution (c). Figures from Bigiel et al. (2008). Right: Disk averaged total gas -star formation law (d). Figure from Kennicutt and Evans (2012).

Fig. 1 .

 1 Fig. 1.13: Molecular gas fraction as a function of redshift. Figure from Tacconi et al. (2020).

Fig. 1 .

 1 Fig. 1.14: Illustration of a datacube.

Fig. 3 . 1 :

 31 Fig. 3.1: Left: Part of the spectrum obtained from the ALMA band 4 observation of AGS4. The yellow shade highlights the detected emission line. The emission line map is shown in the bottom-left corner. The contours denote 1 to 5 ‡, with the step of 1 ‡. Right: Same as on the left, but for AGS17. The contours in the map denote 1 to 6 ‡, with the step of 1 ‡.

Fig. 3 . 2 :

 32 Fig. 3.2: Upper limit of the [CI](2-1)/CO(7-6) flux ratio of AGS4 (blue arrows), compared with existing observations of local and high-redshift MS galaxies, starburst galaxies and AGNs (data compiled in Valentino et al. (2018); Valentino et al. (2020)). If the detected emission line is indeed CO(7-6), then the flux ratio would fall at the lower end of the trend shown by the existing observations.

Fig. 3 . 3 :

 33 Fig. 3.3: Top-left: The SED of AGS4. Top: Photometric redshift probability distribution function derived by EAzY (Brammer et al., 2008). Bottom: The SED fittings. The green curve shows the best-fit of AGS4 at z phot = 4.02 using EAzY, and we shift the SED to the spectroscopic redshift (zspec = 3.556) of AGS4 in light blue for comparison. The grey curve shows the SED of the optically-bright neighbor, ID ZFOURGE = 12333. Top-right: The SED of AGS15. The blue curve shows the best-fit at z phot = 3.96 derived by EAzY and we shift the SED to the redshift peak of the over-density, at z = 3.472, in light blue for comparison. The grey curve shows the SED of the optically-bright neighbor, ID CANDELS = 3818. Bottom-left: The SED of AGS24. Same as AGS15. Bottom-right: The SED of AGS25 at z phot = 4.70, fitted by EAzY with the photometry retrieved from the ZFOURGE catalog.

Fig. 3 . 4 :

 34 Fig. 3.4: Images of AGS4 from the observed B (435 nm, rest-frame UV) to near-infrared observed Ks (2.2 µm, rest-frame B) bands. The red contours denote the ALMA detection at 1.1mm at the resolution of 0 ÕÕ . 29. The green dashed contours denote the ALMA detection at 2mm at the resolution of 0 ÕÕ . 88. The astrometric corrections between ALMA and HST images are applied throughout this work. The cyan "x" denotes the optical bright neighbor, ID CANDELS = 8923 (z phot = 0.24). The magenta "x" denotes the detection by ZFOURGE based on the Ks-band image, ID ZFOURGE = 12333 (z phot = 3.76). If not stated otherwise, the images in this paper are all oriented with the North up and East to the left.

Fig. 3 . 5 :

 35 Fig. 3.5: Images of AGS25 in the H-band (1.6 µm), Ks-band (2.2 µm) and IRAC1-band (3.6 µm). The contribution of a 3 ÕÕ distant bright neighbor of AGS25 was subtracted to produce the IRAC image as in F20a. The red contours denote the ALMA detection at 1.1mm at the resolution of 0 ÕÕ . 60. The magenta "x" is the ZFOURGE counterpart ID ZFOURGE = 11353.

Fig. 3 . 6 :

 36 Fig. 3.6: Redshift distribution of the 11674 galaxies with a redshift (either photometric or spectroscopic) located within the 6.9 Õ ◊10 Õ GOODS-ALMA field (grey). The sub-sample of 395 galaxies located within 50 ÕÕ of the most massive galaxy in the field at z > 3, AGS24, are shown in blue. The insert shows the distribution of the spectroscopic redshifts of the 121 galaxies at 3 AE zspec AE 4 over the GOODS-ALMA field. Ten galaxies fall in the redshift bin 3.470 AE zspec AE 3.480. The mean redshift of the ten galaxies is zspec = 3.472. The redshifts are from the ZFOURGE (Straatman et al., 2016) and VANDELS DR3 (McLure et al., 2018; Pentericci et al., 2018) catalogs.

Tab. 3 . 1 :

 31 Properties of the optically dark galaxies.

( 11 )

 11 Flux densities at 1.1 mm: Peak fluxes measured using Blobcat as in F18 and the fluxes of AGS15 and AGS17 are updated in F20a; (12) Peak fluxes at 2 mm from the ALMA band4 follow-up; (13) CO luminosities: the CO(6-5) line detected in our ALMA spectroscopic scan follow-up;(14) H-band AB magnitudes: After de-blending, AGS4 and AGS15 turn out to have H-band magnitudes higher than the detection limit; (15) Flux densities at 3GHz: the flux densities of AGS4, 17 and 24 are the peak flux derived using PyBDSF 1(Rujopakarn et al. in prep.), flux densities of AGS11, 15 and 25 are the peak flux densities in the 3GHz image;

Fig. 3 . 7 :

 37 Fig. 3.7: Images of AGS24 in the B, V, H and Ks band, IRAC 3.6 and 4.5µm, JVLA 6 GHz (3.7 ‡) and 3 GHz(5.7 ‡). The contours are the same as in Fig.3.4 but for AGS24. The three neighbor galaxies are labeled with ID ZFOURGE or ID CDFS and their redshift separately.

Fig. 3 .

 3 Fig. 3.8: Top: Images of AGS11 in the J band, H band, Ks band and IRAC 3.6 µm . The magenta "x" denotes the detection in Ks-band by ZFOURGE, ID ZFOURGE = 7589 (z phot = 4.82). Bottom: Same as above, but for AGS17. The cyan "x"s denote the optical bright neighbors, ID CANDELS = 4414 (z phot = 0.03, below) and ID CANDELS = 4436 (z phot = 0.95, above). The magenta "x" denotes the detection in Ks-band by ZFOURGE, ID ZFOURGE = 6964 (z phot = 1.85).

Fig. 3 . 9 :

 39 Fig. 3.9: Same as in Fig. 3.4, but for AGS15. The cyan "x" denotes the optical bright neighbor, ID CANDELS = 3818 (z phot = 4.27). The magenta "x" denotes the detection in Ks-band by ZFOURGE, ID ZFOURGE = 6755 (z phot = 3.47).

Fig. 3 .

 3 Fig. 3.10: Local over-density (") of individual galaxies in the GOODS-ALMA field at 3.42 < z < 3.57. The black dots show the sliding medians of " at different stellar masses. The filled circles indicate all the ALMA detections. The purple circles are the HST dark galaxies and the red circle indicates AGS24.

Fig. 3 . 11 :

 311 Fig. 3.11: Projected density of 364 galaxies (40 with zspec) at 3.42 AE z AE 3.57. The contours show 1,2,3,4 ‡ level of the over-density. Left panel covers the field observed in GOODS-South ALMA. Right panel shows the zoom-in of the extended structure in the bottom right corner of GOODS-ALMA field, which encompasses 126(18) galaxies over 15 arcmin 2 . The red star indicates the position of AGS24. The red circle indicates a 50 ÕÕ radius region (2 arcmin 2

Chapter 3

 3 transition and are z AGS4 spec = 3.556 and z AGS17 spec = 3.467.54 GOODS-ALMA: optically-dark ALMA galaxies shed light on a cluster in formation at z = 3.5Existing spectroscopic confirmation of optically dark sources have been so far limited to a few sources, e.g., z = 5.183:[START_REF] Walter | Evidence for Low Extinction in Actively Star-forming Galaxies at z > 6.5[END_REF]; z = 3.717:Schreiber et al. (2018); z = 3.097 & 5.113: Wang et al. (2019).

Fig. 4

 4 Fig. 4.1: Left: CO J=2-1 emission and 1.3 mm continuum superimposed on HST V -band image. The red contours denote the CO J=2-1 detection at the resolution of 1 ÕÕ 71 ◊ 1 ÕÕ 48, starting from 2 ‡, in increments of 1 ‡ (3.3 mJy/beam) significance. The yellow contours denote the continuum detection at 1.3 mm at the resolution of 2 ÕÕ 21 ◊ 2 ÕÕ 06, starting from 1 ‡, in increments of 1 ‡ significance (0.024 mJy/beam). Dashed contours are in negative. Right:The CO J=2-1 spectra with a velocity resolution of 3 km s ≠1 from the merged, obs-17 and obs18 data cubes, extracted from the area defined by the 3- ‡ contour as shown in Figure 4.1-Left. The 1 ‡ uncertainty is 0.72, 0.72, 1.39 mJy, respectively. The grey dash line denotes the marginal detection at ≥46 km s ≠1 .

Fig. 4 . 2 :Chapter 4 Fig

 424 Fig. 4.2: SED of IZw18, with CIGALE best fit adopting a delayed star formation history, a Draine et al. (2007) dust model, and a radio component. A modified blackbody fit is shown with a grey dotted line. The 1.3 mm continuum 3 ‡ upper limit is shown as in the black box.

Fig. 4

 4 Fig. 4.4: L CII /L CO(1≠0) ratio as a function of metallicity (Left) and specific SFR (Right). The Pearson correlation coefficients of the regression fits are shown in the bottom left corners in each panel.

Fig. 5 . 1 :

 51 Fig. 5.1: Images of different components of each galaxy. Top: SFR surface density (extinction corrected) in M §yr ≠1 kpc ≠2 . Middle: ionised gas velocity in km s ≠1 . Bottom: ionised gas velocity dispersion in km s ≠1 . In all images, we only display spaxels with S/N > 34 (see Section 5.2.2.2 for the choice of S/N). The red crosses are the centres of the galaxies. The white solid lines are the major axes (see Section 5.2.2.1).

Chapter 5 Fig. 5 . 2 :

 552 Fig. 5.2: Dependence of gas velocity dispersion ( ‡gas) on velocity gradient (v grad , Equation 5.1). Points in each individual galaxy are labeled by colour. The solid line refers to ‡gas = 2 v grad and the dashed line refers to ‡gas = v grad . The dotted line denotes the velocity gradient at 20 km s ≠1 . These three different lines represent the three different criteria that we tested to account for beam smearing (Section 5.2.2.3).

Fig. 5 . 3 :

 53 Fig. 5.3:Spatially resolved dependence of ‡gas on SFR . We remove pixels with v grad > 0.5 ‡gas to account for beam smearing effects (see Figure5.2 and Section 5.2.2). Our eight SAMI galaxies are compared toLehnert et al. (2009, Figure 12 therein). Each filled circle refers to one spaxel in each galaxy and is colour-coded with the magnitude of velocity gradient (v grad , Equation5.1). The crosses and asterisks in different colours refer to the 11 actively star-forming galaxies at z ≥ 2 in[START_REF] Lehnert | Physical Conditions in the Interstellar Medium of Intensely Star-Forming Galaxies at Redshift˜2[END_REF]. The solid black curves show ‡ Ã (' Ė) 1/2 , where Ė is the energy injection due to star formation, and ' is the coupling efficiency of the energy injected into the ISM. The dashed curves show ‡ Ã (' Ė) 1/3 assuming that velocity dispersions correspond to energy dissipation due to turbulent motions. The red solid curve shows the velocity dispersion of a 10 8 M § clump assuming a simple Jeans relation. A zoom-in of our SAMI galaxies with a logarithmic y-axis is also shown here. All of the models have included the typical thermal broadening of H-of 12 km s ≠1 . The error bars in the zoom-in figure show the maximum range of the thermal broadening of 10 -15 km s ≠1 . The colourbar on the left shows the magnitude of the velocity gradient (vg).

Fig. 5 . 4 :

 54 Fig.5.4: Global dependence of ‡gas on SFR . Our eight SAMI galaxies compared to local high H-luminosity galaxies from[START_REF] Green | DYNAMO -I. A sample of H--luminous galaxies with resolved kinematics[END_REF] and z > 1 star-forming galaxies and clumps (see Section 5.3.2.2 for further details). Each blue (red) filled diamond shows one entire galaxy in our sample including (excluding) the contribution from thermal broadening ( ‡ thermal ≥ 12 km s ≠1 ,[START_REF] Glazebrook | The Dawes Review 1: Kinematic Studies of Star-Forming Galaxies Across Cosmic Time[END_REF]. For the measurement of ‡gas and SFR , see footnotes in Table6.1. Green diamonds refer to the H-luminous galaxies in[START_REF] Green | DYNAMO -I. A sample of H--luminous galaxies with resolved kinematics[END_REF]. The black filled squares, dark green triangles, red open squares and blue open circles refer to the z > 1 star-forming galaxies and clumps. The grey contour denotes the distribution of local star-forming galaxies with gas velocity dispersion derived from HI[START_REF] Dib | The Supernova Rate-Velocity Dispersion Relation in the Interstellar Medium[END_REF], and we include the intrinsic thermal broadening of 12 km s ≠1 here.

Fig. 5 . 5 :

 55 Fig. 5.5: Velocity dispersion as a function of velocity. Spaxels along the major axis are plotted with errorbars.Points in each individual galaxy are labeled by colour.

Tab. 6 . 1 :

 61 Properties of our four EMP galaxies along with additional two galaxies from the

Fig. 6 . 1 :

 61 Fig. 6.1: The Herschel images of our galaxies at 70, 160, 250 and 350 µm from left to right. The large ellipses indicate the extension of the whole star-forming disk and small circles are those individual dusty starforming regions.

Tab. 6 . 3 :

 63 Photometry Of Individual Star-forming Regions In Metal Poor Galaxies

Fig. 6 .Chapter 6 Fig. 6 .

 666 Fig.6.2: Left: The distribution of EMP star-forming regions, and integrated measurements of galaxies from DGS and KINGFISH in the f70µm/f160µm vs. f160µm/f250µm color-color diagram. The grid is a single modified black-body with a range of temperatures and emissivity indices. The solid line represents a constant emissivity index, -, with the temperature ranging from 10 to 100 K in a decrement of 5 K from the left-bottom to the upper-right. Dashed lines represent constant temperature withranging from 0.0 to 3.0 in a step of 0.5 from the left-bottom to the upper-right. The brown line is the trend of star forming galaxies predicted by the model of[START_REF] Dale | A Two-parameter Model for the Infrared/Submillimeter/Radio Spectral Energy Distributions of Galaxies and Active Galactic Nuclei[END_REF]. Right: Same as left but using f160µm/f250µm vs.f250µm/f350µm.

Fig. 6 . 3 :

 63 Fig. 6.3:The two modified black-body fitting to the SEDs. Reds dots are the flux density at different wavelength within each star-forming region. Black solid lines are the 2T MMB best fits with blue dotted lines the best fits of the warm dust components and blue dashed lines the best fits of the cold dust components.

Fig. 6 . 4 :

 64 Fig. 6.4: The fraction of warm dust emission at 100 µm as a function of the Oxygen abundance based on the two modified black-body fitting. The red diamonds are the EMP star-forming regions, and blue circles are for the integrated dwarf galaxies from the DGS.

  Fig. 6.5:The IR colors as functions far-UV surface brightness, 24 µm surface brightness, SFR surface densities and stellar mass surface densities from left to right.

Chapter 6

 6 Fig. 6.6:The dust-to-stellar mass ratio of our EMP star-forming regions and integrated galaxies of the DGS.The lines are the model predictions by[START_REF] Feldmann | The equilibrium view on dust and metals in galaxies: Galactic outflows drive low dust-to-metal ratios in dwarf galaxies[END_REF]. tISM is the dust growth time-scale which can be computed from basic collision theory[START_REF] Weingartner | Interstellar Depletion onto Very Small Dust Grains[END_REF] (solid or dashed lines). QMS denotes the (multiplicative) offset of a given galaxy from the main sequence (blue: 1/3, cyan: 1, green: 3). The data for IZw 18 and SBS 0335-052 are from[START_REF] Hunt | ALMA observations of cool dust in a low-metallicity starburst, SBS 0335-052[END_REF].

  Figure from Kennicutt and Evans (2012). . . . 1.13 Molecular gas fraction as a function of redshift. Figure from Tacconi et al. (2020). . . . . . 1.14 Illustration of a datacube. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Left: Part of the spectrum obtained from the ALMA band 4 observation of AGS4. The yellow shade highlights the detected emission line. The emission line map is shown in the bottom-left corner. The contours denote 1 to 5 ‡, with the step of 1 ‡. Right: Same as on the left, but for AGS17. The contours in the map denote 1 to 6 ‡, with the step of 1 ‡. . . . 3.2 Upper limit of the [CI](2-1)/CO(7-6) flux ratio of AGS4 (blue arrows), compared with existing observations of local and high-redshift MS galaxies, starburst galaxies and AGNs (data compiled in Valentino et al. (2018); Valentino et al. (2020)). If the detected emission line is indeed CO(7-6), then the flux ratio would fall at the lower end of the trend shown by the existing observations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Top-left: The SED of AGS4. Top: Photometric redshift probability distribution function derived by EAzY (Brammer et al., 2008). Bottom: The SED fittings. The green curve shows the best-fit of AGS4 at z phot = 4.02 using EAzY, and we shift the SED to the spectroscopic redshift (z spec = 3.556) of AGS4 in light blue for comparison. The grey curve shows the SED of the optically-bright neighbor, ID ZFOURGE = 12333. Top-right: The SED of AGS15. The blue curve shows the best-fit at z phot = 3.96 derived by EAzY and we shift the SED to the redshift peak of the over-density, at z = 3.472, in light blue for comparison. The grey curve shows the SED of the optically-bright neighbor, ID CANDELS = 3818. Bottom-left: The SED of AGS24. Same as AGS15. Bottom-right: The SED of AGS25 at z phot = 4.70, fitted by EAzY with the photometry retrieved from the ZFOURGE catalog. . . . . . . . . . 126 List of Figures

  and VANDELS DR3 (McLure et al., 2018; Pentericci et al., 2018) catalogs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

  Images of AGS11 in the J band, H band, K s band and IRAC 3.6 µm . The magenta "x" denotes the detection in K s -band by ZFOURGE, ID ZFOURGE = 7589 (z phot = 4.82). Bottom: Same as above, but for AGS17. The cyan "x"s denote the optical bright neighbors, ID CANDELS = 4414 (z phot = 0.03, below) and ID CANDELS = 4436 (z phot = 0.95, above). The magenta "x" denotes the detection in K s -band by ZFOURGE, ID ZFOURGE = 6964 (z phot = 1.85). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.9Same as in Fig. 3.4, but for AGS15. The cyan "x" denotes the optical bright neighbor, ID CANDELS = 3818 (z phot = 4.27). The magenta "x" denotes the detection in K s -band by ZFOURGE, ID ZFOURGE = 6755 (z phot = 3.47). . . . . . . . . . . . . . . . . . . . . . . . . . 3.10 Local over-density (") of individual galaxies in the GOODS-ALMA field at 3.42 < z < 3.57. The black dots show the sliding medians of " at different stellar masses. The filled circles indicate all the ALMA detections. The purple circles are the HST dark galaxies and the red circle indicates AGS24. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . density of 364 galaxies (40 with z spec ) at 3.42 AE z AE 3.57. The contours show 1,2,3,4 ‡ level of the over-density. Left panel covers the field observed in GOODS-South ALMA. Right panel shows the zoom-in of the extended structure in the bottom right corner of GOODS-ALMA field, which encompasses 126(18) galaxies over 15 arcmin 2 . The red star indicates the position of AGS24. The red circle indicates a 50 ÕÕ radius region (2 arcmin 2 ) including 26(

  of a galaxy cluster at z = 2.506, CL J1001+0220(Wang et al., 2016a). . . . . . . . . . . . . . . . . . . . . . . .4.1Left: CO J=2-1 emission and 1.3 mm continuum superimposed on HST V -band image.The red contours denote the CO J=2-1 detection at the resolution of 1 ÕÕ 71 ◊ 1 ÕÕ 48, starting from 2 ‡, in increments of 1 ‡ (3.3 mJy/beam) significance. The yellow contours denote the continuum detection at 1.3 mm at the resolution of 2 ÕÕ 21 ◊ 2 ÕÕ 06, starting from 1 ‡, in increments of 1 ‡ significance (0.024 mJy/beam). Dashed contours are in negative.Right:The CO J=2-1 spectra with a velocity resolution of 3 km s ≠1 from the merged, obs-17 and obs18 data cubes, extracted from the area defined by the 3- ‡ contour as shown in Figure4.1-Left. The 1 ‡ uncertainty is 0.72, 0.72, 1.39 mJy, respectively. The grey dash line denotes the marginal detection at ≥46 km s ≠1 . . . . . . . . . . . . . . . . . . . . . . . 4.2 SED of IZw18, with CIGALE best fit adopting a delayed star formation history, a Draine et al. (2007) dust model, and a radio component. A modified blackbody fit is shown with a grey dotted line. The 1.3 mm continuum 3 ‡ upper limit is shown as in the black box. . . .

  . . . . . . . . . . . . . . . . . . . . . . . . . 128 List of Figures

L

  CII /L CO(1≠0) ratio as a function of metallicity (Left) and specific SFR (Right). The Pearson correlation coefficients of the regression fits are shown in the bottom left corners in each panel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

  All of the models have included the typical thermal broadening of H-of 12 km s ≠1 . The error bars in the zoom-in figure show the maximum range of the thermal broadening of 10 -15 km s ≠1 . The colourbar on the left shows the magnitude of the velocity gradient (v g ). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5 . 5

 55 Velocity dispersion as a function of velocity. Spaxels along the major axis are plotted with errorbars. Points in each individual galaxy are labeled by colour. . . . . . . . . . . . . . . 6.1 The Herschel images of our galaxies at 70, 160, 250 and 350 µm from left to right. The large ellipses indicate the extension of the whole star-forming disk and small circles are those individual dusty star-forming regions. . . . . . . . . . . . . . . . . . . . . . . . . . .

  2

  3). The data for IZw 18 and SBS 0335-052 are from Hunt et al. (2014). . . . . . . . . . . . . . . . . . . . . . . . . . . Titre : Sonder les processus qui conduisent à la formation d' étoiles locales et distantes dans les galaxies à travers la poussi ère et les mol écules Mots cl és : évolution des galaxies, formation d' étoiles, milieu interstellaire, ALMA, Herschel, NOEMA R ésum é : Les études sur l' évolution des galaxies ont ét é révolutionn ées au cours des derni ères d écennies gr âce aux nouveau t élescopes et aux instruments de pointe. Les interf érom ètres Herschel, ALMA et NOEMA permettent d'observer l' émission des poussi ères tracant la formation des étoiles, invisible dans les bandes infrarouges proche observ ées par les t élescopes optiques.

  Assumed redshifts: We assign the redshift of AGS11, AGS15 and AGS24 to be the same as the central position of the redshift peak that presents an over-density centered on AGS24, z peak = 3.472, as discussed in section 3.5.2 and 3.5.3, which is consistent with their photometric redshifts. For each galaxy, all the properties are measured assuming the redshift listed here;

	(8) (9) (10) (11) (12) (13) (14) (15) (16) (17)	AGS4 53.148839 -27.821192 -3.556 3.556 11.09 +0.06 ≠0.18 12.93±0.02 1435 +76 ≠83 3.8 +1.9 ≠0.5 1.72±0.20 0.29±0.02 4.95±0.93 25.23 17.28±1.66 8.64±0.77 2.73◊10 24 +0.75 AGS11 53.108818 -27.869055 --3.472 10.24 ≠0.00 12.94±0.07 1492 +252 ≠253 28.7 +0.1 ≠23.6 1.34±0.25 0.30±0.02 --4.48±1.06 -6.72◊10 23 +0.80 AGS15 53.074847 -27.875880 3.96 ≠0.78 -3.472 10.56 +0.01 ≠0.41 12.78±0.08 1034 +180 ≠179 9.5 +11.2 ≠1.7 1.21±0.11 0.32±0.02 -27.11 5.38±1.06 -8.08◊10 23 +0.40 AGS17 53.079374 -27.870770 -3.467 3.467 10.52 ≠0.06 13.08±0.02 2070 +112 ≠117 20.9 +3.1 ≠12.6 2.30±0.44 0.35±0.02 5.22±1.01 -39.00±3.94 1.16±0.32 5.86◊10 24 +0.49 AGS24 53.087178 -27.840217 3.58 ≠0.38 -3.472 11.32 +0.02 ≠0.19 12.31±0.11 353 +80 ≠82 0.6 +0.5 ≠0.1 0.88±0.22 0.19±0.02 --12.43±2.19 1.41±0.32 1.87◊10 24 +126 AGS25 53.183710 -27.836515 4.64 -4.64 10.39 12.68±0.07 832 ≠135 8.0 +1.3 ≠1.3 0.82±0.19 ---3.92±1.06 1.07±0.32 5.90◊10 23	Columns: (1) Source names; (4) Photometric redshifts: see Section 3.2.4.1; (5) Spectroscopic redshifts: The spectroscopic redshifts of AGS4 and AGS17 are derived from the CO(6-5) line measured in our (2) (3) Coordinates in the ALMA image (J2000): see F18 and F20a;	(8) Infrared luminosities: see Section 3.2.4.3; (9) SFRs: see Section 3.2.4.4; (6) (7) Stellar masses: see Section 3.2.4.2; ALMA spectroscopic scan follow-up, see Section 3.3;	(10) Starburstness: As in F20a, R

SB = SFR/SFR

MS , where SFR MS is the average SFR of MS galaxies as defined in

Schreiber et al. (2018)

;

  Information of the neighbors of AGS24.

	Tab. 3.2: ID ZFOURGE10672	zspec -	z phot 3.53	Mı [10 10 ◊ M § ] [M § yr ≠1 ] SFR 5.88 30.2
	ZFOURGE10627 3.478	3.48	2.63	14.8
	CDFS007493	3.482	-	0.41	-

3.5.3.1 AGS24, the most massive galaxy at z > 3 in GOODS-South

  Stellar masses of AGS24 assuming different SFHs and attenuation laws.

	Tab. 3.3: FAST++ 4 :
	Calzetti et al. (2000) attenuation law + vary SFHs
	Models	log Mı [log M § ]
	delayed exponentially declining exponentialy declining truncated a second burst CIGALE:	11.37 +0.01 ≠0.23 11.31 +0.11 ≠0.19 11.40 +0.00 ≠0.28 11.33 +0.04 ≠0.18
	delayed exponentially declining SFH + vary attenuation laws
	Models Calzetti et al. (2000)	log Mı [log M § ] 11.52±0.57
	Charlot and Fall (2000)	11.41±0.21
	MAGPHYS a	11.44±0.22
	a A modified Charlot and Fall (2000) law that is used in MAGPHYS (da
	Cunha et al., 2008)

  Spectral resolution. Full width half maximum (FWHM) = c/R.

	Tab. 5.1: Red and blue data cubes from LZIFU.
	data cube	⁄ †	R ‡	‡ ú
	Blue	3700 -5700 Å 1730 74 km s ≠1
	Red	6250 -7350 Å 4500 29 km s ≠1

† Wavelength range. ‡ ú Velocity resolution according to spectral resolution.

  29 km s ≠1 ); 2 ‡ gas d ‡ gas = 2 ‡ obs d ‡ obs ;

	dividing both sides with ‡ 2 gas :
	d ‡ gas ‡ gas	=	d ‡ obs ‡ obs	‡ 2 obs gas ‡ 2	;
	rearranging this equation and substituting ‡ 2 obs with ‡ 2 gas + ‡ 2 instr : ‡ 2 obs ‡ 2 gas = ‡ 2 M gas + ‡ 2 instr ‡ 2 gas = ‡ obs d ‡ obs ‡ gas d ‡ gas © (S/N) obs (S/N) true ;
	‡ obs d ‡ obs	=	‡ gas d ‡ gas

  Exposure time of Herschel data used in this paper.

	Tab. 6.2: Name	PACS	SPIRE
		70, 160 µm 250, 350, 500 µm
	SextansA	1.9 hr	6 min
	ESO146-G14	1.6 hr	6 min
	DDO 68	2.7 hr	10 min
	6.2.2 Observations
	Herschel broad-band images (PI: Y. Shi, PID: OT2_yshi_3) of Sextans A, ESO 146-G14 and DDO 68
	were taken at 70 and 160 µm through the scan map modes of PACS

  Pixel sizes of Herschel data used in this paper.

	55±3 32±3 Wavelength 111±4 52±4 164±5 89±4 70, 160 µm Herschel/SPIRE 250, 350, 500 µm 4 arcsec, 6 arcsec, 8 arcsec 18 arcsec, 25 arcsec, 36 arcsec 62±3 34±3 29±4 16±3 28±3 11±3 49±6 31±4 <5 <5 <5 <5 7 ±1 <5 79±4 51±4 205±4 103±4 345±7 196±6 247±6 105±5 Tab. 6.4: Instrument Pixel size Resolution Herschel/PACS 1arcsec, 2 arcsec 5 arcsec, 13 arcsec Spitzer/IRAC 3.6, 4.5µm 0.75 arcsec a , 0.75 arcsec 2.5 arcsec, 2.5 arcsec Spitzer/MIPS 24µm 1.5 arcsec 6 arcsec GALEX/FUV 1516 Å 1.5 arcsec 4.5 arcsec
	56±7 a The pixel size of the 3.6 image of ESO146-G14 is 0.6arcsec. 147±18 297±24 69±8 38±6 52±8 57±9 <2 9 ±1 8 ±1 144± 14 407± 39 630± 19 387± 16
	165±5	497±5	751±8	892±7
	-04:40:27.0 22x22 960 ±0.4 1.64±0.009 1.05±0.008 1.06±0.14 41±2 SextansA/sf-2 10:11:10.0 -04:41:44.5 22x22 660 ±0.4 1.65±0.009 1.36±0.008 3.22±0.33 72±3 SextansA/sf-3 10:11:6.20 -04:42:22.5 32x32 4300 ±0.6 2.78±0.013 2.20±0.011 6.30±0.64 267±4 SextansA/sf-4 10:10:55.5 -04:42:59.4 22x22 260 ±0.4 1.14±0.009 0.69±0.008 0.94±0.13 21±2 ESO146-G14/sf-1 22:13:6.0 -62:03:32.5 10x10 150 ±0.2 0.33±0.003 0.24±0.003 1.02±0.16 28±4 ESO146-G14/sf-2 22:13:2.5 -62:03:51.6 10x10 260 ±0.2 0.38±0.003 0.28±0.003 1.23±0.18 36±5 ESO146-G14/sf-3 22:12:59.0 -62:04:14.3 10x10 90 ±0.2 0.74±0.003 0.50±0.003 0.91±0.16 15±2 DDO68/sf-a 09:56:46.6 +28:50:16.6 10x10 340 ±0.1 0.08±0.004 0.08±0.006 0.71±0.08 11±1 DDO68/sf-b 09:56:46.2 +28:49:39.6 10x10 370 ±0.1 0.41±0.004 0.25±0.006 0.36±0.05 10±1 DDO68/sf-c 09:56:45.3 +28:49:22.6 10x10 350 ±0.1 0.60±0.004 0.43±0.006 0.33±0.05 10±1 HoII/sf-1 08:18:48.5 +70:44:40.1 28x28 880 ±0.5 1.34±0.008 1.09±0.012 16.69±1.67 319±6 HoII/sf-2 08:19:13.3 +70:42:56.3 28x28 2610 ±0.5 20.41±0.008 14.19±0.012 46.32±4.63 471±7 17.91±1.80 449±7 HoII/sf-3 08:18:57.2 +70:42:48.4 46x46 4740 ±0.8 12.31±0.014 63.18±6.32 765±7 8.40±0.020 HoII/sf-4 08:19:26.9 +70:42:27.0 43x43 4780 ±0.7 10.44±0.013 a 7.99±0.018 Star-forming regions we defined in our EMP galaxies; b RA & DEC of the center of each region; c Major axis and minor axis of each region in arcsecond; d Flux density at far-UV of each region, the same with f 3 .6 µm , f 4 .5 µm , f 70 µm , f 160 µm , f 250 µm , f 350 µm .
		6.2 Sample, observations and data analysis

  Results of Single MBB Fits to the SEDs at Ø 100 µm

	Tab. 6.5: Region	T	-	M dust
		[K]		[M §]
	SextansA/sf-1	19± 8	0.99±1.22	1.34◊10 3
	SextansA/sf-2	25± 9	1.02±0.78	1.31◊10 3
	SextansA/sf-3	37± 17	0.59±0.56	9.93◊10 2
	SextansA/sf-4	21± 9	1.00±1.12	1.21◊10 3
	ESO146-G14/sf-1	23± 22	0.90±2.24	1.00◊10 5
	ESO146-G14/sf-2	30± 5	1.14±0.64	4.51◊10 4
	ESO146-G14/sf-3	22± 16	0.80±1.94	2.11◊10 5
	HoII/sf-1	47± 13	0.07±0.44	1.98◊10 3
	HoII/sf-2	37± 4	0.70±0.24	6.03◊10 3
	HoII/sf-3	33± 1	0.82±0.06	1.30◊10 4
	HoII/sf-4	41± 1	0.95±0.06	5.42◊10 3

  And many thanks to all my friends, my teachers and my family back in China, what I want to say to you are included in the Chinese version of my thesis. Thanks Mum and Dad for your unconditional support of all time.Merci Anoine. Mille mercis pour ton grand soutien lors de ma thèsis et l'écriture de ce manuscript. Merci d'être toujours là et de me motiver. Merci d'être venu dans ma vie. Spectral energy distributions (SEDs) for typical galaxies -an old elliptical galaxy (red), two types of spiral galaxies (Sb in green and Sd in blue), an AGN (Markarian 231, solid black), a QSO (dotted black), and a merging and star-bursting galaxy Arp 220. Template spectra are taken from Polletta et al. (2007). Courtesy: CANDELS blog. b The spectral energy distributions (SEDs) of a typical star-forming galaxy (H-ATLAS J090713.1-000322). The total SED is in grey, with the stellar component in red and the corresponding dust emission in blue. The green curve denotes the unattenuated stellar light. Figure from Smith et al. (2012). c: Example of SED fitting -an old, dust free galaxy (blue) and a young, dusty galaxy (red) both fit the observed data points (green) in the UV to mid-infrared (MIR) regime. Courtesy: CANDELS blog. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 The SED of a typical Lyman break galaxy at z ≥ 3, and the images of this galaxy taken with photometric filters at U Cosmic SFRD derived from the combination of the UV data after dust attenuation correction and IR data. Figures from Madau and Dickinson (2014). . . . . . . . . . . . . . The observed flux densities for a typical 10 12.5 L § infraredluminous galaxy as a function of redshift. The curves are color coded with the observed wavelength. Figure from Casey et al. (2014). . . . . . . . . . . . . . . . . . . . . . . . . . ≠2 sr ≠1 is marked in the boxes. Figure from Dole et al. (2006). . . . . . . . . Histograms of the effective radii for the ALMA 870µm continuum emission, stellar mass profiles, and H-band emission from the same work. ÕÕ 60, reaching an rms of 0.18 mJy beam ≠1 ([START_REF] Franco | GOODS-ALMA: Using IRAC and VLA to probe fainter millimeter galaxies[END_REF]. The white and blue circles with a diameter of 4 ÕÕ indicate the blind detections with S/N above the 4.8 ‡ and detections with priors at 3.5 ≥ 4.8 ‡, respectively. Black contours show the different slices (labeled A to F) used to construct the homogeneous 1.1 mm coverage. Blue lines indicate the HST/ACS field and the green lines indicate the HST-WFC3 deep field. The cyan contours represent the limit of the[START_REF] Dunlop | A deep ALMA image of the Hubble Ultra Deep Field[END_REF] survey covering all of the Hubble Ultra Deep Field region, the yellow contours show the ASAGAO survey[START_REF] Hatsukade | ALMA twenty-six arcmin 2 survey of GOODS-S at one millimeter (ASAGAO): Source catalog and number counts[END_REF], the gray contours show the ASPECS Pilot survey[START_REF] Walter | ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: Survey Description[END_REF], the pink contours show the ASPECS Large Program[START_REF] Decarli | The ALMA Spectroscopic Survey in the HUDF: CO Luminosity Functions and the Molecular Gas Content of Galaxies through Cosmic History[END_REF]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.10 Left: Contribution of optically dark galaxies to the cosmic SFRD. Figure from Wang et al. (2019). Right: Upper panel: SFR density for all galaxies (black), protoclusters (blue), and cores (red). Lower panel: Fractional contributions to the total cosmic SFRD. Figure from Chiang et al. (2017). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.11 Illustration of "CO dark" gas in molecular clouds. Figure from Madden et al. (2020). . . . .

	1.1 a: 1.6 Illustration of K-correction: 1.7 Illustration of the SED the Cosmic Microwave Background (CMB), the Cosmic Infrared Background (CIB) and the Cosmic Optical Background (COB). Their approximate bright-ness in nW m 1.8 Left: SFR as a function of stellar mass of the SMGs at 1.7 < z < 2.6 studied by Lang et al. (2019). The solid and dotted lines indicate the main sequence and the 0.3 dex scatter from Whitaker et al. (2014). Right: 1.9 GOODS-ALMA 1.1mm image tapered at 0

n , G, R s bands. Courtesy: CANDELS blog. . . . . . . . . . .

1.3

Left: The galaxy absolute magnitude-color diagram in SDSS filter colors, where absolute magnitude has been converted to stellar mass. Figure adapted from Schawinski et al. (2014) by S. Majewski. Right: Schematic representation of the main sequence and other galaxies with different star formation modes. Courtesy: CANDELS blog. . . . . . . . . . 1.4 Left: The evolution of star formation main sequence with redshift. The average SFR of star-forming galaxies as a function of redshift and stellar mass are parameterized as in Eq. 1.1, see also Schreiber et al. (2015). Right: Double Gaussian decomposition of the distribution of starburstiness (R SB = SFR/SFR MS ). Figures from Schreiber et al. (2015). . . 1.5 Cosmic star formation rate density (SFRD). a: Cosmic SFRD before dust attenuation correction. b: Mean dust extinction in magnitude as a function of redshift. c: Cosmic SFRD derived from the UV data after dust attenuation correction and from IR, respectively. d: Figures from Lang et al. (2019). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.12 Left: Spatially resolved atomic (a), molecular (b) and total (d) gas -star formation law and gas column density distribution (c). Figures from Bigiel et al. (2008). Right: Disk averaged total gas -star formation law (d).

  . Right: Same as left but using f 160µm /f 250µm vs. f 250µm /f 350µm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 The two modified black-body fitting to the SEDs. Reds dots are the flux density at different wavelength within each star-forming region. Black solid lines are the 2T MMB best fits with blue dotted lines the best fits of the warm dust components and blue dashed lines the best fits of the cold dust components. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4

It can be redshifted to longer wavelength as will be discussed later.

"red" and "blue": In astronomy, we define "color" as the magnitude difference of a galaxy through two different photometric filters. Conventionally, it is the magnitude of the filter at the shorter wavelength minus the other one. Then the higher the value of the color, the redder we assume the galaxy is, otherwise, it is blue.

This is not always true as red spiral galaxies and blue ellipticals do exist, rare though (e.g.,van den Bergh, 1976;[START_REF] Moran | GALEX Observations of "Passive Spirals" in the Cluster Cl 0024+17: Clues to the Formation of S0 Galaxies[END_REF][START_REF] Rowlands | Herschel-ATLAS/GAMA: dusty early-type galaxies and passive spirals[END_REF].1.2 Star formation obscured by interstellar dust

Lyman break is a sharp decrease in the SED at ⁄ AE 912 Å. Energetic photons with E >13.6 keV, the ionization energy of a hydrogen atom, emit at ⁄ AE 912 Å. They can be are easily absorbed by the hydrogen gas from the ISM within the galaxy or from the IGM outside the galaxy, hence hardly observable in the SED.

4000 Å break is caused by the blanket absorption of high energy radiation from metals in stellar atmospheres and by a deficiency of hot, blue stars. It coincides with the Balmer break at 3646 Å which arises from a similar mechanism to the one of Lyman break, but the energetic photons at ⁄ > 3646 Å ionize the first excitation state of hydrogen atom instead of ground state hydrogen atom. It is an indicator of stellar population age[START_REF] Kauffmann | Stellar masses and star formation histories for 10 5 galaxies from the Sloan Digital Sky Survey[END_REF] 4Chapter 1 Introduction

That's why conventionally elliptical galaxies are called "early-type" galaxies and spiral galaxies are called "late-type" galaxies.1.2 Star formation obscured by interstellar dust

L

is the luminosity at 8 µm dominated by the emission from PAH molecules.1.2 Star formation obscured by interstellar dust

Chapter 1 Introduction

This peak wavelength tends to shift a bit to shorter wavelength at higher redshift because of the higher T dust as mentioned in Section 1.2.2.

The luminosity distance D L can be approximated as D L Ã (1 + z) 2 at 0.5 < z < 3, and D L Ã (1 + z) 1.5 at z > 3. Given that dust emissivity has a typical value of -= 1.5 -2, the observed flux density barely changes with redshift.

The observed flux density S‹ = L‹ /4fiD 2 à (1 + z) -+2 /(1 + z) 4 à (1 + z) -≠2 at 0.5 < z < 3 or à (1 + z) -≠1.5 at z > 3.

1.3 Star formation fueled by gas

Chapter 1 Introduction

Codes available at https://editeodoro.github.io/Bbarolo/.

Codes available at https://supercomputing.swin.edu.au/portfolio/gbkfit/.

Codes available at http://galpak3d.univ-lyon1.fr/.20Chapter 1 Introduction

This work has been published in Astronomy & Astrophysics[START_REF] Zhou | GOODS-ALMA: Optically dark ALMA galaxies shed light on a cluster in formation at z = 3.5[END_REF], and is presented in Chapter 3.

Chapter 2 Summary of the work done in this thesis

2.1 GOODS-ALMA

2.3 The SAMI Integral field Units (IFU) Survey

Chapter 3 GOODS-ALMA: optically-dark ALMA galaxies shed light on a cluster in formation at z = 3.5

https://www.astron.nl/citt/pybdsf/3.2 Data and observations

VANDELS DR3: http://archive.eso.org/cms/eso-archive-news/new-data-release-of-spectra-and-catalog-from-vandels. html; http://vandels.inaf.it/db/query_catalogs.jsp.

By applying the de-blending method described in[START_REF] Schreiber | Jekyll Hyde: quiescence and extreme obscuration in a pair of massive galaxies 1.5 Gyr after the Big Bang[END_REF], also see code developed by Corentin Schreiber: https://github.com/cschreib/qdeblend.

FAST++: https://github.com/cschreib/fastpp, a version of the codeFAST Kriek et al. (2009) fully rewritten that can handle much larger parameter grids and offers additional features.3.2 Data and observations

3.3Results of the ALMA spectroscopic follow-up

3.4 GOODS-ALMA optically-dark galaxies

3.5 An over-density at z ≥ 3.5 in GOODS-ALMA

Chapter 3 GOODS-ALMA: optically-dark ALMA galaxies shed light on a cluster in formation at z = 3.5

Here we adopt solar metallicity as 12 + log(O/H) = 8.7(Asplund et al.,

2009)

Chapter5 The SAMI Galaxy Survey: energy sources of the turbulent velocity dispersion in spatially-resolved local

Extinction-corrected SDSS DR7 Petrosian mag.5.2 Sample and data analysis

which includes -a) z ≥ 2 star-forming clumps[START_REF] Genzel | The Sins Survey of z ˜2 Galaxy Kinematics: Properties of the Giant Star-forming Clumps[END_REF] as black filled squares; b) z ≥ 2-3 low mass (≥ 10 9 M § ) lensed star-forming galaxies[START_REF] Jones | Resolved spectroscopy of gravitationally lensed galaxies: recovering coherent velocity fields in subluminous z ˜2-3 galaxies[END_REF] as dark green triangles; c) z ≥ 1.5-3 low mass (≥ 0.3-3 ◊10 10 M § ), compact but well-resolved with adaptive optics "dispersion dominated" star-forming galaxies[START_REF] Law | Infrared Luminosity Functions from the Chandra Deep Field-South: The Spitzer View on the History of Dusty Star Formation at 0 < ~z < ~1[END_REF] as red open squares; d) z ≥ 1.5-2.5 disks from the SINS survey[START_REF] Cresci | The SINS Survey: Modeling the Dynamics of z ˜2 Galaxies and the High-z Tully-Fisher Relation[END_REF][START_REF] Schreiber | The SINS Survey: SINFONI Integral Field Spectroscopy of z ˜2 Star-forming Galaxies[END_REF] as blue open circles.

† H-luminous galaxies in[START_REF] Green | DYNAMO -I. A sample of H--luminous galaxies with resolved kinematics[END_REF]. ‡ z >1 star-forming galaxies and star-forming clumps. See Section 5.3.2.2 for further detail. ú Significance level of the Spearman correlation coefficient.5.3 Results

[START_REF] Engelbracht | Metallicity Effects on Mid-Infrared Colors and the 8 µm PAH Emission in Galaxies[END_REF] adopted Solar metallicity as 12 + log(O/H) = 8.7(Allende[START_REF] Prieto | The Forbidden Abundance of Oxygen in the Sun[END_REF]. Here in this paper, we define Solar metallicity to be 8.7 as well. Thus one-third of the Solar metallicity is around 8.2.

Chapter 6 Spatially resolved dust emission of extremely metal poor galaxies 4

6.2 Sample, observations and data analysis

Temperatures are derived from modified blackbody, with the emissivity indexfixed to be 2.

http://www.astro.princeton.edu/≥draine/dust/dustmix.html, (Milky Way, R_V = 3.1) 90 Chapter 6 Spatially resolved dust emission of extremely metal poor galaxies

6.4 Dust-to-stellar mass ratio

Chapter 6 Spatially resolved dust emission of extremely metal poor galaxies12

Compared to star-forming galaxies at Solar metallicity, metal poor galaxies show higher L IR /L Õ CO(1≠0) and SFR/L Õ CO(1≠0) ratios. The L IR /L Õ CO(1≠0) ratio of metal poor galaxies is on average 15.4 times higher than the one of the massive, normal star-forming galaxies at Solar luminosity. IZw18 shows even higher L IR /L Õ CO(1≠0) and SFR/L Õ CO(1≠0) ratios, which are & 5.7 and & 119 times higher than those of other metal poor galaxies (Fig. 4.3-Top left), respectively. CO molecules are hard to form in metal poor environments because there is fewer raw material and there is little dust to protect the already scarce CO molecules from UV radiation. The higher ratios are a natural result of this. The SFR/L Õ CO(1≠0) ratio of metal poor galaxies increases as the metallicity decreases (Fig. 4.3-Bottom right). Hunt et al. (2015a) found a significant correlation when fitting their dwarf galaxies, detections in [START_REF] Schruba | Low CO Luminosities in Dwarf Galaxies[END_REF] and the compilation therein. However, when including all galaxies in the regression and comparing with IZw 18, we see a shallower trend, especially in the star-forming in the extremely metal poor galaxies (Shi et al., 2015a;[START_REF] Shi | Carbon monoxide in an extremely metal-poor galaxy[END_REF], we see a shallower trend. On the other hand, the SFR/L Õ CO(1≠0) ratio of IZw18, which is at 12 + log(O/H) = 7.17 (Izotov and Thuan, 1999a), increases dramatically and does not follow the trend defined by the other galaxies. This suggests a drastic change in the CO distribution/content of galaxies at a few percentages of Solar metallicity.

SFR and L Õ

CO(1≠0) can be derived from observations, but the molecular gas mass, M H2 = -CO ◊ L Õ CO(1≠0) , is the key quantity of interest. The observed SFR/L Õ CO(1≠0) ratio is related to both the depletion time (• dep ) and the CO-to-H 2 conversion factor (-CO ) as SFR/L Õ CO(1≠0) = SFR/(M H2 /-CO ) = -CO /• dep . Many studies have shown that -CO increases rapidly as low metallcity theoretically and observationally [START_REF] Israel | H_2 and its relation to CO in the LMC and other magellanic irregular galaxies[END_REF][START_REF] Glover | On the relationship between molecular hydrogen and carbon monoxide abundances in molecular clouds[END_REF][START_REF] Leroy | The CO-to-H2 Conversion Factor from Infrared Dust Emission across the Local Group[END_REF][START_REF] Narayanan | A general model for the CO-H2 conversion factor in galaxies with applications to the star formation law[END_REF][START_REF] Elmegreen | Carbon monoxide in clouds at low metallicity in the dwarf irregular galaxy WLM[END_REF][START_REF] Shi | Carbon monoxide in an extremely metal-poor galaxy[END_REF]. If • dep is constant as is found for the nearby disk galaxies [START_REF] Leroy | The Star Formation Efficiency in Nearby Galaxies: Measuring Where Gas Forms Stars Effectively[END_REF][START_REF] Bigiel | A Constant Molecular Gas Depletion Time in Nearby Disk Galaxies[END_REF], then the dependence of SFR/L Õ CO(1≠0) ratio on metallicity is the direct consequence of the variation of -CO . However, • dep in dwarf galaxies remains uncertain and tends to be longer at lower mass and at lower specific SFR (sSFR; [START_REF] Saintonge | COLD GASS, an IRAM legacy survey of molecular gas in massive galaxies -II. The non-universality of the molecular gas depletion time-scale[END_REF]Shi et al., 2014a). IZw18, as a dwarf galaxy with low sSFR, is likely to have higher • dep . This, together with the high SFR/L Õ CO(1≠0) ratio of IZw18, would lead to an even higher -CO , as

. Further constraints on the conversion factor (-CO ) is beyond the scope of this paper.

4.4.3

The structure of the interstellar medium [START_REF] Bolatto | A Semianalytical Model for the Observational Properties of the Dominant Carbon Species at Different Metallicities[END_REF] and [START_REF] Röllig | CII] 158 µm emission and metallicity in photon dominated regions[END_REF] modeled the photodissociation regions (PDRs) and found that as the [CII] regions expand at low metallicity, the L [CII] /L CO(1≠0) ratio increases, which is confirmed in observations [START_REF] Madden | Effects of massive star formation on the ISM of dwarf galaxies[END_REF][START_REF] Cormier | The molecular gas reservoir of 6 low-metallicity galaxies from the Herschel Dwarf Galaxy Survey. A ground-based follow-up survey of CO(1-0), CO(2-1), and CO(3-2)[END_REF]. [START_REF] Cormier | The Herschel Dwarf Galaxy Survey. I. Properties of the low-metallicity ISM from PACS spectroscopy[END_REF] detected the bright [CII] emission in IZw18. We plot L [CII] /L CO(1≠0) ratio as a function of metallicity and specific SFR in Fig. 4.4. IZw18 generally follows the trend defined by the DGS and xCOLD galaxies but shows ratios several times higher than the prediction of the regression fits. [START_REF] Cormier | The Herschel Dwarf Galaxy Survey. II. Physical conditions, origin of [C II] emission, and porosity of the multiphase low-metallicity ISM[END_REF] modeled the mid and far-IR emission of the DGS galaxies and found that the covering factor of PDRs decreases at decreasing metallicity which indicates an increase of the ISM porosity. Then the extreme case of IZw18 reinforces the drastic change of the structure of the ISM in systems at a few percentages of Solar metallicity and may have similar implications for such systems in the early Universe.

List of Tables

Colophon

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