Kazuki Tokuda
email: tokuda@p.s.osakafu-u.ac.jp

Yasuo Fukui

Ryohei Harada

Kazuya Saigo

Kengo Tachihara

Kisetsu Tsuge

Tsuyoshi Inoue

Kazufumi Torii

Atsushi Nishimura

Sarolta Zahorecz

Omnarayani Nayak

Margaret Meixner

Tetsuhiro Minamidani

Akiko Kawamura

Norikazu Mizuno

Remy Indebetouw

Marta Sewi

Suzanne Madden

Maud Galametz

Vianney Lebouteiller

C.-H Rosie Chen

Toshikazu Onishi

An ALMA view of molecular filaments in the Large Magellanic Cloud II: An early stage of high-mass star formation embedded at colliding clouds in N159W-South

Keywords: stars, formation -stars, protostars -ISM, clouds-ISM, kinematics and dynamics -ISM, individual (N159W)

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INTRODUCTION

Though high-mass stars are considered to have great impacts on the galaxy evolution, their formation process is not fully understood. There are a number of works that investigated the high-mass star formation mechanism (for reviews, e.g., Zinnecker & York 2007;[START_REF] Tan | Protostars and Planets VI[END_REF]. Although the precursors of massive stars are supposed to be very dense and massive (∼100 M ⊙ ) cores (e.g., Krumholz et al. 2009;Peretto et al. 2013), it may be a problem how such peculiar cores are formed as an initial condition of massive star formation. Habe & Ohta (1992) suggested that molecular cloud collisions form a dense shock compressed layer, which is massive enough to form a massive star based on their numerical simulations in a short timescale less than 1 Myr (see also, Takahira et al. 2014;Matsumoto et al. 2015;Shima et al. 2018). Inoue et al. (2018) demonstrated that shock compressions induced by cloud-cloud collision promote massive filament formation, which is perpendicular to the background magnetic field (see also Inoue & Fukui 2013). O-type stars are formed by the global collapse of the massive filament with a high mass-accretion rate, >10 -4 M ⊙ yr -1 .

Because the high-mass protostars are supposed to be formed in giant molecular clouds (GMCs), detailed observational studies toward the GMCs are thus needed to examine the initial conditions of high-mass star formation. Large-scale surveys with high-angular resolution (<0.1-1 pc) in the Galaxy have been providing us with fruitful knowledge on the physical properties of molecular clouds and (high-mass) star formation. Recent high-angular resolution observations with ground-based single-dish telescopes and the Herschel satellite revealed that filamentary structures are ubiquitous in both dark clouds and GMCs (e.g., Mizuno et al. 1995;Nagahama et al. 1998;Onishi et al. 1996Onishi et al. , 1999;;Arzoumanian et al. 2011Arzoumanian et al. , 2019;;[START_REF] André | Protostars and Planets VI[END_REF][START_REF] André | [END_REF]. High-mass young stellar objects (YSOs) tend to be located at the intersection of multiple filamentary clouds, called "hub filament" (e.g., Myers 2009;Peretto et al. 2013). The central part of hub filaments is as massive as 1000 M ⊙ pc -1 , which is sometimes referred to as a "ridge" (e.g., Motte et al. 2007;Hill et al. 2012;Nguyen-Lu'o'ng et al. 2013). Motte et al. (2018) and references therein suggested that such massive filaments are supposed to be products of the global hierarchical collapse of molecular clouds. Because the line mass of the ridges is significantly higher than the critical line mass of an isothermal filament (see Inutsuka & Miyama 1997), by up to two orders of magnitude, they are supposed to be unstable against the global collapse and fragmentation. Although some formation/stabilization mechanism of the hub filaments and the ridges, such as large-scale compression and internal MHD (magneto-hydrodynamic) waves, have been proposed [START_REF] André | [END_REF]), the true nature is not fully understood, possibly due to the lack of the suitable targets in the solar neighborhood.

ALMA is capable of resolving internal structures of molecular clouds even in external galaxies. In particular, the Large Magellanic Cloud (LMC) is an ideal laboratory to investigate high-mass star formation thanks to its nearly faceon view (Balbinot et al. 2015) and the close distance, ∼50 kpc (Schaefer 2008;de Grijs et al. 2014). It is also a great advantage to directly compare the distributions of molecular gas observed by ALMA and positions of massive YSOs identified by Spitzer and Herschel (e.g., Gruendl & Chu 2009;Chen et al. 2010;Seale et al. 2014) without any serious contamination in the line of sight. Earlier studies using the H i gas observations by Fukui et al. (2017) found that there are supergiant shells (Kim et al. 1999(Kim et al. , 2003) ) and kiloparsec-scale gas flows caused by the last tidal interaction between the LMC and the Small Magellanic Clouds (SMC). Therefore, we may be able to examine the relation between such large-scale gas kinematics and the local star formation activities. Our present target in this paper is the N159W-South clump, which was discovered by our previous ALMA Cycle 1 observations (Fukui et al. 2015, hereafter Paper I) with an angular resolution of ∼1 ′′ (∼0.24 pc) toward a GMC in the N159W region (e.g., Johansson et al. 1998;Minamidani et al. 2008Minamidani et al. , 2011)). Paper I revealed that the GMC is composed of many filamentary molecular clouds and discovered the first example of protostellar outflows in the external galaxies. Paper I also found that the protostellar source with a stellar mass of ∼37 M ⊙ in the N159W-South clump is located toward an intersection of two filaments and suggested that the filament-filament collision triggered the protostar formation. Although the ALMA observations significantly improved our understanding of molecular cloud structures and star formation in this object, much higher-angular-resolution studies are needed to further resolve the filamentary structures down to a width of 0.1 pc (see, Arzoumanian et al. 2011Arzoumanian et al. , 2019) ) and investigate the star formation activities therein.

In this paper, we present high-angular resolution observations obtained in ALMA Cycle 4 (P.I.: Y. Fukui #2016.1.01173.S) with ∼0. ′′ 25 (∼0.07 pc) resolution toward the N159W-South clump. We also observed the N159W-North and the N159E-Papillon region in the same project. The observational results of the N159E-Papillon region, which is considered to be in a later evolutionary stage than the N159W-South clump (Saigo et al. 2017), are presented in a separate paper (Fukui et al. 2019, hereafter FTS19).

We carried out ALMA Cycle 4 Band 6 (211-275 GHz) observations toward the N159W with the main array 12 m antennas. The observations of N159W-South centered at (α J2000.0 , δ J2000.0 ) = (5 h 39 m 41. s 0, -69 • 46 ′ 06. ′′ 0) were carried out between 2016 November and 2017 July. There were three spectral windows targeting 12 CO (J = 2-1), 13 CO (J = 2-1) and C 18 O (J = 2-1) with a bandwidth of 58.6 MHz. The frequency resolutions were 30.6 kHz for 12 CO (J = 2-1) and 61.0 kHz for the others. We used two spectral windows for the continuum observations with the aggregated bandwidth of 3.75 GHz. The observed frequencies include some line emission, such as the radio recombination line of H30α and SiO (J = 5-4). The projected baseline length ranges from 14 to 1940 m.

The data were processed with the CASA (Common Astronomy Software Application) package [START_REF] Mcmullin | Astronomical Data Analysis Software and Systems XVI[END_REF]) version 5.0.0. We used the tclean task in the imaging process with the multi-scale deconvolver to recover the extended emission. The imaging grid and velocity channels were 0. ′′ 07 and 0.2 km s -1 , respectively, and we applied the Briggs weighting with the robust parameter of 0.5. We used the auto-multithresh procedure in tclean to select the emission mask in the dirty and residual images. We continued the deconvolution process until the intensity of the residual image reached ∼1σ noise level. The synthesized beams of the continuum and the 13 CO (J = 2-1) observations are 0. ′′ 26 × 0. ′′ 23 and 0. ′′ 29 × 0. ′′ 25, respectively. The (1σ) RMS noise levels of the line and the 1.3 mm continuum are ∼4.5 mJy beam -1 (∼1.5 K) at a velocity resolution of 0.2 km s -1 and ∼0.027 mJy beam -1 . We concluded that the previous ALMA Cycle 1 observations (Paper I, see also Nayak et al. 2018) did not show significant missing flux of this source based on a comparison between the ALMA and the single-dish observations. The total fluxes of the 12 CO, 13 CO, and the 1.3 mm continuum data of the Cycle 4 were ∼20% lower than those of the Cycle 1. Because the current observations fully cover the baseline range of the previous one, 16-395 m, the discrepancies are supposed to be mainly caused by the calibration error instead of the missing flux. We thus use the Cycle 4 data alone in the following analysis.

RESULTS

A dense filamentary cloud traced by the 1.3 mm dust-continuum emission associated with molecular outflows

Figures 1 (a-c) show 1.3 mm dust continuum distributions toward the N159W-South clump. Although the previous low-resolution (∼1 ′′ ) data shown in the panel (a) identified the clump as a single-source with a north-south elongation (see also Paper I), the present Cycle 4 high-resolution observations clearly resolved the filamentary structure with multiple local maxima. We named the four major continuum maxima as MMS-1, 2, 3, and 4 labeled in Figure 1 (c). The positions of MMS-1 and MMS-2 correspond to near-infrared (NIR) sources, #121 and #123, respectively [START_REF] Testor | [END_REF]). These sources were originally discovered as a single source (P2) by Jones et al. (1986), and the subsequent high-resolution observations with the VLT (Very Large Telescope) resolved it into more than two sources [START_REF] Testor | [END_REF], see also the Gemini observations by Bernard et al. 2016). In the K s -band observations, the two NIR sources have almost the same magnitude (13.59 mag for #121, and 13.40 mag for #123), and are the brightest ones and highly reddened among the Testor's sample. [START_REF] Testor | [END_REF] concluded that the two sources are considered to be YSOs. MMS-3 and 4 have no counterparts in the infrared observations. The sources detected by [START_REF] Testor | [END_REF] were in the K s magnitude range of 13.40-19.4 mag. Although this means that the embedded sources in MMS-3 and 4 are much fainter than the known NIR sources, the presence of the outflow activities (see the later paragraph) strongly indicates that there are YSOs in the millimeter-sources. We derived the physical properties of the millimetersources as listed in Table 1. We assumed the absorption coefficient per unit dust mass at 1.3 mm, and the dust-to-gas mass ratio and the dust temperature to be 1 cm 2 g -1 , 3.5 × 10 -3 , and 20 K, respectively, to derive the gas mass from the dust emission (see, Ossenkopf & Henning 1994;Herrera et al. 2013;Gordon et al. 2014;Paper I). The separations among the dust peaks are roughly ∼0.2 pc.

We characterized the properties of the 1.3 mm filamentary clouds. The total mass of the filamentary structure traced by the continuum above the 3σ detection is ∼4 × 10 3 M ⊙ , which is consistent with the previous estimate (Paper I). We searched the peak positions (i.e., crests of the filament) in the R.A. direction along each decl. bin of Figure 1 (c). The length of the filament crests is ∼1.7 pc. We performed the Gaussian fitting to the cross section taken along the filament crest. The averaged width (FWHM) of the filament is ∼0.14 pc. The resultant line mass (=total mass/length) of the filament is ∼2 × 10 3 M ⊙ pc -1 . These properties are consistent with those of "ridges" in Galactic high-mass star-forming clouds (e.g., NGC 6334, [START_REF] André | [END_REF].

We have detected compact high-velocity wings in the 12 CO (J = 2-1) observations tracing outflows from the protostellar sources. We identified the outflow components as follows. Based on the 12 CO channel map, we searched highvelocity emission more than 10 km s -1 apart from the systemic velocity (∼236 km s -1 ) by eye, and then we extracted Table 1.

Properties of 1.3 mm continuum sources in the N159W-South clump Name α(J2000.0) δ(J2000.0) Fν (mJy) a Fmax (mJy beam -1 ) N peak (cm -2 ) b M (M⊙) c NIR source d
MMS-1 5 h 39 m 41. s 97 -69 b H2 column density at the peak position.

c Total mass integrated above the white dashed lines in each source.

d Name of associated near infrared sources [START_REF] Testor | [END_REF].

the spectra of the high-velocity emission. We manually selected the lowest velocities of the redshifted/blueshifted outflows to avoid the strong emission of the parental filamentary clouds. Using the velocity-smoothed data, we defined the highest velocities where the intensities of the wing components are close to zero levels. The red and blue contours in Figure 1 (b) show the distributions of the outflow components, and the averaged spectra in each outflow are shown in Figure 2. For the sources in Figure 1 (c), the previous 12 CO observations identified these multiple flows as a single bipolar flow due to the lack of the angular resolution (Figure 2 in Paper I). The blue/red wings from MMS-1 and 2 are possibly merged, as shown in Figure 1 (b). Especially for the blue component, it is hard to separate the individual components. We plotted the same spectra in the Figures 2 (d), (e) and listed as the blue wing of MMS-1. Although the redshifted components around the MMS-1 and 2 are also spatially merging, there are multiple local peaks in the contours. The presence of at least two individual YSOs (#121, and #123) in each millimeter source indicate that the spatially merging blue/redshifted lobes are originated from the protostars. Nevertheless, higher-angular-resolution observations in CO will be able to clearly resolve the individual outflow components. On the other hand, the outflows around MMS-2 and 3 are well spatially separated, indicating that they are individual outflows launched by the embedded YSOs. Note that the broadening of the 12 CO linewidths toward the millimeter sources is considered to be originate from not only the outflowing gas but from cloud-cloud collision. We discuss the latter effect in Sect. 3.2.

In addition to the above, we discovered a new candidate of bipolar outflow ∼2 pc away from the N159W-South clump (Figures 1(b) and 2(a)). The green crosses indicate the position of the center of the high-velocity gas in Figures 1 (a,b). This strongly indicates that there is at least one embedded protostar; we hereafter call this source YSO-SN. Although this source is located in the 13 CO filamentary cloud, as shown in Figure 3, we could not detect any 1.3 mm continuum emission with the present sensitivity. This suggests that the parental core is less massive than the other millimeter sources in the N159W-South clump. We need high-sensitivity and high-angular resolution observations in millimeter/submillimeter continuum or high-density molecular gas tracers to characterize the nature of the parental core of YSO-SN.

We estimated the physical parameters of these outflows and have listed them in Table 2. In order to derive the masses of the outflows, we adopted a conversion factor from 12 CO (J = 1-0) intensity to the column density of X CO = 7 × 10 20 cm -2 (K km s -1 ) and the 12 CO (J = 2-1)/ 12 CO (J = 1-0) ratio = 1.0, with the case of a gas kinematic temperature of ∼30 K and a volume density of ∼10 4 cm -3 (Beuther et al. 2002b). We measured the projected distances to the peak intensity of the outflow lobes from the continuum peaks MMS-1, 2, 3, and 4. For YSO-SN, we adopted the length of the blueshifted and redshifted lobes as the distance. Using the maximum velocity with respect to the systemic velocity measured from the 13 CO spectra (Figure 2), we estimated the dynamical time (t d = distance/velocity) of the flow by assuming an inclination angle of 30 • -70 • . We calculated the outflow force (F CO ) by using the procedure in Beuther et al. (2002b). The relations between the F CO and the core mass derived from the dust emission for MMS-1, 2, 3 and 4 roughly follow those seen at massive protostellar sources in the Galaxy (Beuther et al. 2002b). Although we could not detect dust emission from YSO-SN, this is also considered to be a massive source because the outflow parameters are very similar to those of the other massive sources. We thus conclude that at least five protostellar sources with separations of ∼0.2-2 pc are forming along the filamentary cloud without significant time delay more than ∼10 4 yr based on the dynamical time of the outflows (Table 2). We also found similar star formation activities in the N159E-Papillon region (FTS19). Note that the directions of the outflows in the N159W-South clump are roughly perpendicular to the orientation of the filament, indicating that the magnetic field directions are also perpendicular to that. We discuss the formation scenario of the YSOs and the filament in Sect. 4.

Velocity and spatial structures of the 13 CO filamentary clouds

Figure 3 shows the 13 CO (J = 2-1) distributions around the N159W-South clump. The overall distributions of both Cycle 1 and 4 data are quite similar, suggesting that the new observations well recover the total flux of the previous one (see Sect. 2). The previous Cycle 1 data represent a relatively simple shape, which is composed of overlapping by the two linear-shaped filaments. In contrast to this, the Cycle 4 data trace further complex substructure of the filaments entangled by the multiple filaments. Looking into the velocity structures as shown in the channel maps of Figure 4, the complexity becomes more apparent. For example, one of the most striking features is hub-like filaments entangled toward the dust filament around the velocity channel of ∼237 km s -1 (dashed lines in the lower left panel in Figure 4). Similar morphologies are seen in the Galactic high-mass star-forming clumps (e.g., Motte et al. 2007;Peretto et al. 2013;Williams et al. 2018). We calculated the column densities using the 13 CO (J = 2-1) data assuming the local thermo-dynamical equilibrium. The "DisPerSE" (Sousbie 2011) algorithm was applied to the 13 CO cube data to search the connecting structures (i.e., filaments) in the PPV space (see also the detailed procedure in FTS19). The column density, line mass, and width (FWHM) of the typical filaments are ∼10 22 -10 23 cm -2 , a few hundred M ⊙ pc -1 , and ∼0.1 pc, respectively, except for the highest column density part with the 1.3 mm continuum detection. These filaments are significantly more massive than those in the solar neighborhood (see Arzoumanian et al. 2011) and close to those in the high-mass star-forming region (e.g., Vela C, Hill et al. 2012).

The velocity analysis of the 13 CO data found that the blueshifted (230.0-233.2 km s -1 ) and redshifted (239.0-242.0 km s -1 ) components have the complementary distributions (Figure 5 (a)). The first-moment intensity-weighted velocity map using the full-velocity range of the 13 CO data also shows a similar trend. Complementary gas distributions are often found in regions with cloud-cloud collision events (e.g, Furukawa et al. 2009;Matsumoto et al. 2015;Torii et al. 2017;Hayashi et al. 2018;Nishimura et al. 2018;Sano et al. 2018;Tokuda et al. 2018;Fukui et al. 2017Fukui et al. , 2018a,b),b). We made position-velocity (PV) diagrams of 13 CO and 12 CO along the 1.3 mm continuum filament to investigate the gas kinematics (Figure 5). The 13 CO PV diagram shows two high-velocity components both in the redshifted and blueshifted sides, as indicated by the green dotted lines in Figure 5 (b). The positions of the highvelocity (V-shaped) features are close to those of star-forming protostellar sources (MMS-1, 2, 3, and 4). Although the V-shaped structures seem to be related to the protostellar activities, they are not explained by the outflows alone.

With respect to the 12 CO PV diagram (Figure 5 (c)), the positions of the high-velocity components due to the outflows indeed correspond to those of millimeter sources. On the other hand, there are position discrepancies between the crests of the V-shaped features in 13 CO and those of the outflows in 12 CO. The spatial extent of the V-shaped features is ∼0.5 pc, which is much broader than that of the outflows, ∼0.1 pc. We interpret that the V-shaped features as originating from the cloud-cloud collision as the trigger of the protostar formation in this region (see also discussions in Sect. 4.2). A similar position discrepancy between a V-shaped feature and an outflow on the PV diagram is also found in the Galactic high-mass star-forming region RCW34 (Hayashi et al. 2018).

4. DISCUSSIONS

Dynamical state of the massive filament in the N159W-South clump

In this section, we discuss the formation scenario of the high-mass protostars and the filamentary molecular clouds in the N159W-South region. The line mass of the filaments in this region is as large as ∼a few × 10 2 -10 3 M ⊙ pc -1 . Such massive filaments should be in a "supercritical" state (see Inutsuka & Miyama 1997), and are considered to fragment and radially collapse within the freefall time. We speculate that the massive filaments that we see in the present observations are formed quite recently and may be gravitationally unstable objects (see also discussions in [START_REF] André | [END_REF]. The typical velocity dispersion (σ v ) of the N159W-South clump is ∼1.7 km s -1 in 13 CO, and the resultant virial mass per unit length (M vir,line = 2σ v /G) is calculated to be 1.3 × 10 3 M ⊙ pc -1 . Note that the magnetic field can be an additional factor to stabilize the filament, depending on the strength as expressed by Inoue et al. (2018). In this case, a strong magnetic filed ( 2 mG) is needed to stabilize the observed massive filament with the line mass of ∼2 × 10 3 M ⊙ pc -1 in the N159W-South clump. This is inconsistent with recent observations toward Galactic massive star-forming regions (e.g., Pillai et al. 2015;Pattle et al. 2018). In summary, we suggest that the turbulent and magnetic pressure may not stabilize the filament.

Assuming the cylindrical shape of the filament with a width of ∼0.14 pc (Sect. 3.1), the averaged density is calculated to be ∼10 6 cm -3 . The freefall time of the gas density is ∼10 4 yr, which is consistent with the lifetime of the starless phase of massive dense cores suggested by the observations toward the Galactic massive star-forming regions (Motte et al. 2018 and references therein). The short dynamical time of the outflows (∼10 4 yr) indicates the embedded YSOs are in an extremely young phase after their formation. In addition, the presence of an outflow source YSO-SN without the 1.3 mm continuum detection may imply that the star formation was initiated before the formation of the parental dense material (i.e., dense core), although further careful investigations are needed to obtain a robust conclusion through follow-up observations, as mentioned in the Sect. 3.1. In summary, we suggest that there is a possibility that the filaments and the protostars in the N159W-South region were formed at almost the same time, within an order of ∼10 4 yr.

Possible formation mechanisms of the massive filament and high-mass protostars

Recent studies of Galactic star-forming regions suggest that massive filaments (ridges) have hub-or web-like structures, which may be a consequence of the global hierarchical collapse (Motte et al. 2018 and references therein). Such an interpretation is based on the gas kinematics traced by molecular line observations showing a parsec-scale (line-of-sight) velocity gradient along with the filamentary structures (e.g., Peretto et al. 2014). In the N159W-South clump, the velocity gradient is small, with a velocity of ∼237 km s -1 on the top side and a quickly changed velocity of ∼235 km s -1 close to the positions of the millimeter sources with outflows (yellow crosses in the dotted rectangle of Figure 5 (a)). Although this type of velocity sequence (i.e., positive-negative-positive) can be regarded as gravitational contraction toward the center of the gravitational potential (e.g., Hacar et al. 2017) if we directly rely on the 2D Hub-filaments Figure 4. Velocity-channel maps toward the N159W-South clump in 13 CO (J = 2-1). The lowest velocities are given in the upper left corners in each panel. The white contours show the 1.3 mm continuum emission as shown in Figure 1. The contour level is 0.1 mJy beam -1 . The angular resolution is given as a white ellipse in the lower left corner of the lower left panel, 0. ′′ 29 × 0. ′′ 25. The white, yellow, and green crosses are the same as those in Figure 1.

velocity map, recent 3D numerical simulation by Li & Klein (2019) cautioned that such an interpretation based on the line-of-sight velocity map could lead to an incorrect conclusion. They demonstrated that this type of velocity map is reproduced by a collision of the gas flow of two clouds and our velocity map in Figure 5 (a) is very similar to the middle panel of Figure 16 in Li & Klein (2019). With respect to the PV diagram of Figure 5 (b), we found two V-shaped features and indicate them with green lines on the blueshifted and redshifted sides around the high-mass star-forming millimeter sources. Numerical simulations (Takahira et al. 2014) and the synthetic observations (Fukui et al. 2018b) of colliding clouds also reproduce similar V-shaped structures in the PV diagram. The present gas characteristics are consistent with the previous cases of the cloud-cloud collision as the trigger of massive star formation. Additonally, Arzoumanian et al. (2018) found a young filament in Taurus with a similar V-shape feature in the PV diagram and they suggested that convergence of flow is a convincing mechanism of the filament formation. In summary, we conclude that the convergent flows or collisions between two clouds are more likely rather than a global hierarchical collapse, to interpret the velocity structure of the 13 CO data in this system. We previously discussed that a collision between two massive filaments a line mass of a few 100 M ⊙ pc -1 triggered the massive star formation, based on the lower-resolution results (Paper I, see also Figure 3 (b)). However, such a simple model of two colliding filaments may be hard to reconcile with the complex hub filaments revealed by the present observations. The complex filaments themselves are considered to have originated from-much larger scale kinematics, such as colliding flows. Based on the observational study in Taurus, Tafalla & Hacar (2015) proposed a "fray and fragment" scenario composed of two steps. The supersonic collision between two flows creates a network of filaments/fibers, and then the filament system fragments into individual cores or groups of cores (chains). Although the mass of the cores in the N159W-South clump is one or two orders of magnitude higher than that in Galactic low-mass star-forming regions, such as Taurus, there are some common features: the separation of individual cores (∼0.1-0.2 pc) and its chain-like morphology, as presented by Tafalla & Hacar (2015). With respect to the outside of the N159W region, there is another star-forming filament complex in the N159E region located at ∼50 pc away from the N159W-South clump (Saigo et al. 2017;FTS19). The N159E region has a compact H ii region, the Papillon Nebula at the edge/intersection of the hub filament, and thus it is slightly more evolved than the N159W-South clump. Nevertheless, there are little observable differences between the two objects in the filament properties and star-formation activities. The filamentary complexes in both the N159W-South and N159E-Papillon regions have conical shapes with a vertex on the south sides and hub-like features around the high-mass protostars, as well as the massive/dense filaments traced by the 1.3 mm continuum emission. In the velocity structures of 13 CO (Figures 4, and5; see also Figures 2 and3 in FTS19), there are velocity gradients that are redshifted and blueshifted along the north-south direction, although the degree of the gradient (∼1 km s -1 pc -1 ) in N159W is little smaller than that of the N159E, possibly due to the projection effect. We also found several outflow activities with a dynamical time scale of ∼10 4 yr along the 1.3 mm continuum filament in both regions. In summary, the N159W-South clump is supposed to be an almost twin of the N159E-Papillon region. To explain the synchronized high-mass star and filament formation over a ∼50 pc scale, we consider a large-scale (>100 pc) triggering event rather than the local ∼10 pc motion around the N159W/E regions.

Filament and massive star formation triggered by colliding flows

A systematic search of large H i structures in the LMC (Kim et al. 1999) found that the N159 region is located along the western edge of a H i supergiant shell, SGS19, which has a radius of ∼390 pc and an expansion velocity of ∼25 km s -1 (Dawson et al. 2013). More recently, Fukui et al. (2017) pointed out that the molecular ridge, the most massive molecular complex in the LMC, contains the N159 region was formed by large-scale H i gas flows, with a relative velocity of ∼50 km s -1 induced by the last galactic tidal interaction between the LMC and the SMC. The presence of the ongoing large-scale collision is also supported by the distribution of the A v map in this region (Furuta et al. 2019). Such a high-velocity H i flow could be a promising candidate as the origin of the strong shock compression that produces the massive filaments and massive protostars in the N159W/E regions.

Inoue et al. ( 2018) performed numerical simulations of colliding clouds with magnetic field and turbulence and demonstrated that the turbulent inhomogeneous cloud is compressed by the shock wave, and hub filaments are developed within a few 0.1 Myr after the collision. The first protostar (sink particle) is created in a few × 10 4 yr after the development of the filaments. A picture of star formation as understood by the theoretical study and the current observations is schematically shown in Figure 6. Although there are no large differences in the dynamical time of the outflows toward each protostellar source of more than ∼10 4 yrs, the evolutionary stages of the northern three sources (MMS-3, 4, and YSO-SN) are supposed to be younger than the others based on the detection of the infrared emission (Figures 1 and6). This distribution is qualitatively explained if the initial small cloud collided with the extended cloud (Inoue et al. 2018) and the star-formation took place as the propagation of the interaction layer. The evolutionary sequence is also consistent with that in the N159E-Papillon region where the compact H ii region is growing at the southern edge of the filamentary clouds (FTS19). According to the simulations by Inoue et al. (2018), a large velocity difference between the colliding clouds of the order of 10 km s -1 is initially needed to promote high-mass star formation activities (see also Inoue & Fukui 2013). However, the density-weighted gas velocity is as small as ∼1-2 km s -1 with respect to the systemic velocity along the star(sink particle)-forming filament, because the shock speed is decelerated in the dense region when the shock wave induced by the collision hits a dense clump (Fukui et al. 2018a; see also Inoue & Inutsuka 2012;Inoue & Fukui 2013). In the N159W-South clump, the mass-weighted velocity (i.e., local peaks on the PV diagram of Figure 5 (b)) around the protostellar sources is also not significantly shifted, not more than a few tens of km s -1 , with respect to the systemic velocity, ∼236 km s -1 . This is consistent with the simulations. We thus suggest that the initial collision velocity might be at least a few × 10 km s -1 based on the H i studies as mentioned in this section, and then, the collision shock velocity was considered to be decelerated in the dense region. In the H i colliding flow scenario, we note that there is an issue regarding the formation time-scale of H 2 molecules if the large-scale H i compression quickly creates the molecular filaments traced by the CO observations. The formation timescale of H 2 molecules from H atoms on dust grains is considered to be as long as ∼10 7 yr, with an atomic gas density of 10 2 cm -2 (Hollenbach & Salpeter 1971;Jura 1974), which is much longer than that of the filament formation as discussed above. Although it is possible, in principle, to make ∼10 4 M ⊙ molecular filaments after the H i gas flow if there is a sufficient mass reservoir around the filamentary cloud (Fukui et al. 2018a), further theoretical and observational investigations are needed to clearly understand the relation between the filament formation and high-velocity H i flows.

Another interesting feature is that the orientations of the observed outflows are roughly perpendicular to that of the filament (Sect. 3.1). This may indicate that the directions of magnetic fields are also perpendicular to the filament if the outflows were launched along the magnetic field direction. Simulations in Inoue et al. (2018) suggest that the magnetic field strength is significantly enhanced at the post-shock layer and massive filaments become perpendicular to the magnetic field. From an observational prespective, for example, Palmeirim et al. (2013) revealed that high-density filaments are perpendicular to the magnetic fields, while low-density striations are parallel in low-mass star-forming filaments in Taurus. Recent ALMA observations by Kong et al. (2019) toward an infrared dark cloud found that large numbers of CO outflows are preferentially orthogonal to the parental filaments and discussed the presence of strong magnetic field as their origin. Future polarization observations toward the N159W-South clump with ALMA may provide us with further evidence of the cloud-cloud collision as the trigger for massive filament/protostar formation in terms of magnetic field.

SUMMARY

We have carried out ALMA observations with an angular resolution of ∼0. ′′ 25 (∼0.06 pc) toward the N159W-South region in the LMC. The 1.3 mm dust continuum traces a clear filamentary feature with a line mass of ∼2 × 10 3 M ⊙ and it has four local peaks with a strong indication of outflow activities. We have identified a new bipolar outflow source embedded at a 13 CO filament, but it is located at ∼2 pc away from the massive dust filament. We have revealed an early stage of multiple high-mass star formation as a few additional outflow sources along the 1.3 mm filamentary cloud. The molecular line observations in 13 CO (J = 2-1) revealed the complex hub structures toward the dust filament, rather than simple linear filaments, as reported in our previous lower-resolution observations. We propose that the massive protostars and filaments are formed from the large-scale flow, which is consistent with the recent theoretical simulations.

Figure 1

 1 Figure 1. 1.3 mm continuum distributions and molecular outflows toward the N159W-South clump. (a) Gray-scale image of the 1.3 mm continuum emission obtained in the previous Cycle 1 study (Paper I). The white crosses represent the positions of the infrared sources identified with the VLT observations (Testor et al. 2006). The yellow and green crosses denote the dustcontinuum peaks of MMS-3 and 4, and the median position between the redshifted and blueshifted outflow lobes shown in panel (b), respectively. (b) The gray-scale image and white contours are the same as (a) but for the high-resolution Cycle 4 data. The minimum contour level and the subsequent steps are 0.1 and 0.2 mJy beam -1 , respectively. Redshifted and blueshifted outflow lobes in 12 CO (J = 2-1) are shown in blue and red contours, respectively. The contour levels are 10, 20, 30, and 40 K km s -1 . The integrated-velocity ranges are shown in dashed lines in panels (a-d) of Figure 2. (c) Zoomed-in view of the N159W-South clump in 1.3 mm continuum showing the filamentary structure. The white dashed lines, corresponding to ∼0.4 mJy beam -1 , represent the boundary of the identified millimeter sources.

Figure 2 .

 2 Figure 2. Outflow spectra toward the N159W-South clump. (a1)-(d1) Red and blue lines of averaged spectra of the outflow wings for MMS-1,2,3,4 and YSO-SN over the regions inside the lowest red and blue contours, respectively, in Figure1 (b). The blue and red dotted lines show the defined velocity ranges of the outflows (see also Table2). Note that the same spectra are plotted as the blue wings in panels (d1) and (e1) (see the text). The red lines are offset by +10 K for the visualization. The gray lines show the 13 CO (J = 2-1) spectra at the positions of MMS-1, 2, 3 and YSO-SN. Black lines indicate the central velocities obtained by fitting the 13 CO spectra with a single Gaussian profile. (a2)-(e2) Same as (a1)-(e1) but for the enlarged views (in Y-axis for (a)-(c), and X-axis for (d) and (e)), to stress the wing features of the outflow spectra.

Figure 3 .

 3 Figure3. 13 CO (J = 2-1) distributions of the N159W-South region. (a) Color scale of the velocity-integrated intensity map of 13 CO (J = 2-1) obtained by the Cycle 4 observations. The white, yellow, and green crosses and white contours are the same as those in Figure1. The angular resolution, 0. ′′ 29 × 0. ′′ 25, is given by the white ellipse in the lower left corner. (b) Same as (a) but for the Cycle 1 data. The angular resolution, 1. ′′ 3 × 0. ′′ 8, is given by the white ellipse in the lower left corner.

Figure 5 .

 5 Figure 5. Velocity structures in 13 CO (J = 2-1) toward the N159W-South clump. (a) The first-moment intensity-weighted velocity map of 13 CO (J = 2-1) is shown in color scale. The blue and black contours show the velocity-integrated intensity of 13 CO (J = 2-1) with a range of 230.0-233.0 km s -1 and 239.0-242.0 km s -1 . The lowest contour level and the subsequent contour step are 3 and 4 K km s -1 , respectively. The white and yellow crosses are the same as those in Figure 1. The angular resolution is given by the ellipse in the lower right corner. (b) A 13 CO (J = 2-1) position-velocity diagram along the regions shown by the dashed rectangle in panel The X-axis represents angular distances in parsecs from the position of MMS-1. The vertical dashed lines in panel (b) represent the position of the millimeter sources with the outflows. The cyan and red horizontal lines show the velocity ranges for the blueshifted and redshifted components, respectively, in panel (a). The green dashed lines stress V-shaped features, possible signs of cloud-cloud collision (see the text). (c) Same as (b) but for 12 CO (J = 2-1).

Figure 6 .

 6 Figure 6. Schematic views of the star formation triggered by a cloud-cloud collision in the N159W-South. A similar figure was presented by FTS19.

Table 2 .

 2 Outflow properties in the N159W-South clumpSource Name Outflow Lobe Mass (M⊙) Distance (pc) Velocity a (km s -1 ) Velocity Range (km s -1 )

	t b	b (yr)

OBSERVATIONS

We thank Doris Arzoumanian and Shu-ichiro Inutsuka for discussions about the filamentary molecular clouds. This paper makes use of the following ALMA data: ADS/ JAO.ALMA#2012.1.00554.S and #2016.1.01173.S. ALMA is a partnership of the ESO, NSF, NINS, NRC, NSC, and ASIAA. The Joint ALMA Observatory is operated by the ESO, AUI/NRAO, and NAOJ. This work was supported by NAOJ ALMA Scientific Research grant No. 2016-03B and JSPS KAKENHI (grant No. 22244014, 23403001, 26247026, 18K13582, 18K13580, and 18H05440). The work of M.S. was supported by NASA under award number 80GSFC17M0002.

Software: CASA (v5.0.0; McMullin et al. 2007)


Yasuo Fukui
email: fukui@a.phys.nagoya-u.ac.jp

Kazuki Tokuda

Kazuya Saigo

Ryohei Harada

Kengo Tachihara

Kisetsu Tsuge

Tsuyoshi Inoue

Kazufumi Torii

Atsushi Nishimura

Sarolta Zahorecz

Omnarayani Nayak

Margaret Meixner

Tetsuhiro Minamidani

Akiko Kawamura

Norikazu Mizuno

Remy Indebetouw

Marta Sewi

Suzanne Madden

Maud Galametz

Vianney Lebouteiller

C.-H Rosie Chen

Toshikazu Onishi

An ALMA view of molecular filaments in the Large Magellanic Cloud I: The formation of high-mass stars and pillars in the N159E-Papillon Nebula triggered by a cloud-cloud collision

Keywords: stars, formation -stars, protostars -ISM, clouds-ISM, kinematics and dynamics -ISM, individual (N159E, the Papillon Nebula)

INTRODUCTION

Obtaining a comprehensive picture of high-mass star formation is one of the most important issues in astrophysics. Although several promising mechanisms of high-mass star formation have been proposed and explored (e.g., Zinnecker & York 2007;[START_REF] Tan | Protostars and Planets VI[END_REF]), these theories are not yet able to fully explain various aspects of high-mass star formation through when compared with observations. We note that many isolated or asymmetric O-star formation (e.g., M42, Fukui et al. 2018a;GM24, Fukui et al. 2018b;RCW34, Hayashi et al. 2018; see also [START_REF] Ascenso | The Birth of Star Clusters[END_REF] and the production of dense and massive cores as the initial condition of high-mass star formation as posited in theories (e.g., Krumholz et al. 2009), for instance, remain important challenges to be addressed by the various star formation theories.

Most recently, supersonic cloud-cloud collisions (i.e., gas compression by collisions between different-velocity gas streams) have been extensively explored and tested as a trigger of high-mass star formation both by theoretical and observational studies (see the PASJ special issue: Star Formation Triggered by Cloud-Cloud Collision (Volume 70, Issue SP2) and the other papers). Among them, the magnetohydrodynamic (MHD) numerical simulations made by Inoue et al. (2018) showed that massive filaments with high-mass stars at their vertex are naturally formed at the shock-compressed layer by a cloud-cloud collision (see also Inoue & Fukui 2013). According to their simulations, there is no significant delay between the formation of the filament and the first sink particle (the high-mass protostar)more than a few 0.1 Myr. Although the intrinsic turbulent motions in a molecular cloud form filamentary structures without any colliding flows, more massive/high-density hub-filamentary structures (see Myers 2009) tend to be created preferentially in the colliding case, as supported by Matsumoto et al. (2015), Balfour et al. (2015), Wu et al. (2017), and Takahira et al. (2018). Hub filaments associated with high-mass protostellar sources are often seen in many Galactic high-mass star-forming regions, such as infrared dark clouds (e.g., Busquet et al. 2013;Peretto et al. 2013;Williams et al. 2018).

Concerning observational aspects, multiple components of molecular clouds are often seen as a possible evidence of cloud-cloud collisions in the Galactic cluster-forming regions (e.g., Furukawa et al. 2009;Fukui et al. 2016;Nishimura et al. 2018). In addition, recent observations toward high-mass star-forming regions with a resolution higher than 0.1 pc have been attempting to resolve filamentary structures, which may be crucial in regulating star formation efficiency (e.g., NGC 6334, [START_REF] André | [END_REF]. Even though the filament width of high-mass star-forming regions is not significantly different from the typical filament width of ∼0.1 pc in the low-mass star-forming regions [START_REF] André | Protostars and Planets VI[END_REF], the line mass (mass per unit length) of filaments is one or two orders of magnitude higher than the critical isothermal line mass (see Inutsuka & Miyama 1997). [START_REF] André | [END_REF] suggests that the massive filaments that we see in the present day may be an indirect piece of evidence for their very recent formation because such super critical filaments are supposed to be unstable against the gravitational collapse. This suggestion is consistent with the theory that massive filaments and high-mass stars are rapidly formed by shock compression that originated in cloud-cloud collisions or converging flows.

Observations toward external galaxies can be very powerful in the study of high-mass star formation studies compared to those in the Milky Way because we can avoid the serious contaminations along the lines of sight toward the galactic plane. However, larger distances generally make it difficult to achieve a spatial resolution high enough to resolve filaments. In this regard, the Large Magellanic Cloud (LMC) is close to us at a distance of ∼50 kpc (Schaefer 2008;de Grijs et al. 2014), and its face-on view (Balbinot et al. 2015) allows us to isolate associations among the molecular clouds, massive YSOs, and H ii regions without the abovementioned contamination problem. Extensive molecular gas surveys toward the LMC identified ∼300 giant molecular clouds (GMCs) (Fukui et al. 1999(Fukui et al. , 2008;;Kawamura et al. 2009;Fukui, & Kawamura 2010) and found that the N159 GMC is a crucial object as the most active ongoing star formation site among the samples.

Our present target, the N159E-Papillon region, is an ideal source to investigate the filament/high-mass star formation as well as the feedback from massive YSOs. The Hα observations with Hubble Space Telescope discovered a butterfly shaped H ii region, "the Papillon Nebula," with the wings separated by ∼0.6 pc (Heydari-Malayeri et al. 1999;Meynadier et al. 2004). The very compact size of the H ii region indicates that the embedded YSO system (hereafter, the Papillon Nebula YSO), which contains two massive protostars with the stellar masses of ∼21 M ⊙ (Heydari-Malayeri et al. 1999;[START_REF] Testor | [END_REF]) and ∼41 M ⊙ (Indebetouw et al. 2004;Chen et al. 2010), is in a very young stage of the onset of the surrounding gas ionization. The previous ALMA Cycle 1 observations with an angular resolution of ∼0.24 pc (Saigo et al. 2017, hereafter Paper I) found that at least three filamentary CO clouds with different velocities are entangled toward the Papillon Nebula, and that the shape of the Papillon Nebula nicely fits the cavity of CO emission. Based on these results, the authors suggested that a filamentary cloud collision triggered high-mass star formation, analogous to N159W. Prior to N159E, Fukui et al. (2015) made a similar analysis of the ALMA Cycle 1 data in N159W and found that two CO filaments are crossed with each other and that a high-mass protostar showing outflow is located toward the crossing position. This was the first protostellar outflow discovered outside the Milky Way, lending further support that protostellar outflows are ubiquitous in protostellar evolution in galaxies. The N159E and N159W regions are separated by ∼50 pc in the sky, which requires 10 Myr to travel at a typical velocity dispersion of ∼5 km s -1 , longer than the protostellar age of 1-5 Myr. However, it remained somewhat curious that the extension of the filamentary clouds is seen toward the same direction in fan shape, to the north of the young stars, both in N159E and N159W (see also Figure 6 of Paper I).

In this paper, we present new ALMA Cycle 4 observations toward the N159E-Papillon region with a factor of three higher angular resolution than the previous study. Section 2 contains descriptions of the observations and in Section 3 containes the results, where we mainly focus on the filamentary molecular clouds and the pillar-like structure around the Papillon Nebula. In addition, YSOs are newly identified, with indications of outflow wings along the filamentary structure. In Sect. 4, we discuss the formation scenario of molecular filaments, high-mass stars, and the pillars. Note that we also observed two other targets (the N159W-South and the N159W-North region) in this project, and the observational results of the N159W-South region are presented in a separate paper (Tokuda et al. 2019, hereafter, TFH19).

OBSERVATIONS

We conducted ALMA observations toward the N159E-Papillon region with Band 6 (211-275 GHz) in Cycle 4 as a part of the multi-objects survey in N159 (P.I. Y. Fukui #2016.1.01173.S.) by using the 12 m array alone. The central coordinate of the observed field toward the N159E-Papillon region was (α J2000.0 , δ J2000.0 ) = (5 h 40 m 04. s 0, -69 • 44 ′ 34. ′′ 0). The target lines were 12 CO (J = 2-1), 13 CO (J = 2-1), C 18 O (J = 2-1), SiO (J = 5-4), and H30α. The observation settings and the data reduction processes are described in TFH19. In the imaging process, we used "tclean", which is implemented in the Common Astronomy Software Application package [START_REF] Mcmullin | Astronomical Data Analysis Software and Systems XVI[END_REF], with a noise threshold of the ∼1σ noise level. We did not apply the self-calibration process. The synthesized beam sizes of the molecular lines ( 12 CO, 13 CO, and C 18 O) are 0. ′′ 28 × 0. ′′ 25, using the Briggs weighting with a robust parameter of 0.5. The sensitivities (RMS noise levels) of the lines are ∼4.0 mJy beam -1 (∼1.4 K) for 12 CO, ∼4.5 mJy beam -1 (∼1.5 K) for 13 CO, and ∼4.5 mJy beam -1 (∼1.2 K) for C 18 O at a velocity resolution of 0.2 km s -1 . The beam size and RMS noise level for the 1.3 mm continuum map are 0. ′′ 26 × 0. ′′ 23, and ∼0.027 mJy beam -1 , respectively. There is no significant missing flux in the present data set compared to our Cycle 1 observations (Paper I). The total fluxes of the 12 CO, 13 CO, and the 1.3 mm continuum data of the Cycle 4 were ∼20% lower than those of the Cycle 1 (see also the detailed flux comparison between the two data sets shown in Appendix A.1). We thus use the Cycle 4 data alone in this paper. Actually, the 13 CO intensity distributions of the new data have reasonably reproduced the overall CO distribution obtained in our previous lower-resolution study (see also Figure 1 in Sect. 3.1).

We made moment-masked cube data (e.g., Dame 2011;Nishimura et al. 2015) to suppress the noise effect in the velocity analyses. We set the emission-free pixels, which are determined by significant emission from the smoothed data in the velocity/spatial axes, as zero values. We used the masked cube data to make moment maps.

RESULTS

13 CO Filaments in the N159E-Papillon region

Figure 1 shows integrated intensities of 13 CO (J = 2-1) toward the N159E-Papillion region obtained in the ALMA Cycle 4 and 1 (Paper I). We found overall agreement with each other, while the Cycle 4 data clearly resolve significantly more details of the filaments. Figure 2 shows the velocity-channel map of the region, which also shows the multiple filamentary distributions. Most of the filaments are aligned roughly in the north-south direction. The first-moment map of 13 CO (J = 2-1) in Figure 3 (a) shows the velocity gradient along that direction. Note that a similar figure was also presented in the previous lower-resolution study (Figure 6(a) in Paper I). The averaged 12 CO, 13 CO, and C 18 O spectra shown in Figure 3 (b) demonstrate that there are no velocity components significantly over the blueshifted ( 225 km s -1 ) and redshifted ( 245 km s -1 ) ranges. This means that there are no external components affecting the determination of the first-moment map (Figure 3 (a)). Figure 3 (c) shows the 13 CO position-velocity (PV) diagram along the dashed rectangle in Figure 3 (a). We estimated the centroid velocity in each X-axis bin with Gaussian fittings, and then the velocity gradient was derived by the least-squares fitting with a linear function within the X-range of -1.8 to 1.0 pc. The velocity gradient is ∼1.5 km s -1 pc -1 and its possible origin will be discussed in Sect. 4.1. , has a remarkable peak at ∼0.1 pc with the standard deviation of 0.08 pc. The widths of most of the filaments are significant at the present resolution (∼0.07 pc). Furthermore, the derived width is not an effect of the averaging of the radial profile across all the identified filaments, as shown in Figure 4 (d). Such a small filamentary width is similar to the Galactic star-forming regions reported by Arzoumanian et al. (2011Arzoumanian et al. ( , 2019) ) and others, and is the first detection of such narrow filaments outside our Galaxy (see also THF18). We estimated the column density and mass of the filamentary clouds using the 12 CO and 13 CO data by assuming the local thermodynamical equilibrium. We obtained the excitation temperature (T ex ) map from the 12 CO data assuming that the line is optically thick. Then we got the optical depth of 13 CO using the T ex map, and finally, the 13 CO column density was derived using the previous calculations. We adopted a 13 CO/H 2 abundance ratio of 3.1 × 10 -7 , taking 12 CO/H 2 as 1.6 × 10 -5 and 12 CO/ 13 CO as 50 (see also Mizuno et al. (2010) and the references therein), to obtain the H 2 column density. The cumulated mass of all structures shown in Figure 1 using the mean molecular weight, µ, of (Kauffmann et al. 2008) is ∼1 × 10 4 M ⊙ , which corresponds to ∼10-20% of that of the entire molecular cloud in the N159E region (Paper I). The averaged column density of the N159E-Papillon region shown in Figure 1 is calculated to be ∼5 × 10 22 cm -2 . The central column densities at the filament crests (Σ 0 ) are ∼(5-30) × 10 22 cm -2 and the resultant line masses (M line ) are calculated to be ÿ1.5 km s -1 pc -1 ∼(1-7) × 10 2 M ⊙ pc -1 using the equation M line ≈ Σ 0 × W fil (e.g., [START_REF] André | Protostars and Planets VI[END_REF]. This is consistent with filaments found in the Galactic high-mass star-forming regions (Hill et al. 2012 for Vela C; [START_REF] André | [END_REF] for NGC 6334). Such filaments with very large line-mass are supposed to be quite unstable against the gravitational collapse unless we assume that there is an additional support force, such as strong magnetic fields (see also discussions in [START_REF] André | [END_REF]. a Maximum brightness temperature of 12 CO (J = 2-1).

CO Pillars around the Papillon Nebula

b Maximum H2 column density.

c Averaged H2 column density within the certain regions. In order to separate from the surrounding structures, the boundaries of Pillar 1, 2, 3, and 5 were defined as 50%, 60%, 70%, 50%, respectively, of the maximum column densities of each pillar. The boundary of Pillar 4 is manually defined at the edge of the white line because we cannot divide the surrounding high-column density gas located at the northern part of the pillar with the similar criterion.

d FWHM of the pillars (see the text).

e Averaged H2 volume density obtained by dividing the Nave by the width. . We define these structures as "CO pillars" by eye based on the abovementioned features, independently of the filament identification (Sect. 3.1), and the CO pillars are shown by white lines in Figure 5 (c). This is the first detection of "pillars of creation" in an external galaxy (see Hester et al. 1996 for the Eagle Nebula). The properties of the CO pillars are listed in Table 1. The averaged column densities of the CO pillars are (0.9-3.0) × 10 23 cm -2 . We derived the width (FWHM) of the pillars by Gaussian fitting to the averaged column density profile perpendicular to the white lines in Figure 5. The resultant width of the pillars is ∼0.1-0.2 pc, which is similar to that of the filaments in the region. The average volume density, n(H 2 ), is ∼(1-7) × 10 5 cm -3 . These results show that the density and temperature are significantly higher than those of the surrounding filaments. The high-density nature is also supported by the detection of the C 18 O (J = 2-1) emission along the CO pillars (Figure 5 (d)). We do not see clear extinctions of the Hα emission along the CO pillars, except for the central part of the Papillon Nebula, despite its high-column density. This indicates that the pillars are embedded in the ionized region or located behind it.

Protostellar sources with molecular outflow

Figure 6 (a) shows the 1.3 mm continuum and C 18 O (J = 2-1) distribution around the Papillon Nebula. The extended 1.3 mm continuum emission around the Papillon Nebula YSO roughly agrees with the distribution of the Hα emission and has no counterparts of molecular gas, indicating that the continuum emission seems to be dominated by free-free emission from the ionized gas (see also Paper I). On the other hand, the filamentary dust clumps along the C 18 O emission, a tracer of cold/dense molecular gas, are considered to be dominated by the thermal dust emission. Two major local maxima, hereafter MMS-1 and MMS-2, are found in the 1.3 mm continuum image in Figures 6 (ac). Because we detected extended emission in 1.3 mm and C 18 O along the north-south direction, the two sources are considered to be dense cores embedded in the filamentary cloud. The peak column densities and total masses of both sources deduced from the 1.3 mm continuum emission are ∼1 × 10 24 cm -2 , and ∼2 × 10 2 M ⊙ , respectively, with an assumption of the dust emissivity, κ 1.3mm of 1 cm 2 g -1 for protostellar envelopes (e.g., Ossenkopf & Henning 1994), a dust-to-gas ratio of 3.0 × 10 -3 (Herrera et al. 2013;Gordon et al. 2014) and a uniform dust temperature of 20 K, which is typically adapted/estimated for galactic high-mass star-forming dust clumps (e.g., Urquhart et al. 2014;Yuan et al. 2017). Note that the core boundaries deriving their total masses were set as the 30% intensity level c The outflow mass is estimated from the 12 CO (J = 2-1) intensity by assuming a conversion factor form 12 CO (J =1-0) intensity to the column density of Xco = 7 × 10 20 cm -2 (K km s -1 ) -1 cm -2 (Fukui et al. 2008) and the 12 CO (J = 2-1)/ 12 CO (J = 1-0) ratio = 1.0.

• • • • • • • • • MMS-1 • • • • • • 4.8/3.4 0.1 12.7/26.7 MMS-2 • • • • • • 2.5/
d Projected distances to the peak intensity of the outflow lobes from the continuum peaks.

e Maximum radial velocity of the outflow lobe with respect to the systemic velocity (see also the black lines in of each continuum peak to avoid the contaminations from the parental filamentary clouds. Although these sources were not cataloged as point sources in the infrared observations so far (e.g., Chen et al. 2010;Carlson et al. 2012), their detection in the 1.3 mm is strongly suggestive of their protostellar nature. We have identified outflow wings that have a velocity span of ∼30 km s -1 in 12 CO (J= 2-1) toward MMS-1 and MMS-2. Figures 6 (b-e) show the distributions and the line profiles of the outflow wings. We extracted the spectra and chose the velocity ranges using the following procedure. We defined the outer boundaries of the outflow spectra (i.e., maximum relative velocity) above the 2σ level of the velocity-smoothed spectra with a smoothing kernel of 9 ch (=1.8 km s -1 ). We extracted the spectra at positions ∼1 ′′ away from the millimeter sources in the east direction as the references without the outflow contaminations and defined the velocities below 2σ intensity levels as the inner edge of the outflow spectra (see Figure 6(d) and (e)). The outflow and YSO properties are listed in Table 2. The size of the outflow is as small as ∼0.1 pc, and escaped detection with our lower-resolution study (Paper I). It is likely that the launching points of the outflows coincide with the 1.3 mm continuum peaks. Because the positional offsets between the blue-shifted and red-shifted lobes are quite small toward MMS-1 and MMS-2, the outflow orientations may be close to pole-on or very small. The dynamical time (t d ) of the outflows is roughly estimated to be <10 4 yr from a ratio of 0.1 pc/20 km s -1 . We roughly estimated the mechanical forces of the outflow lobes (F out ) using the following equation: F out = M out V out /t d (e.g., Beuther et al. 2002), where V out and M out are the outflow velocity and mass, respectively. The estimated F out is ∼10 -3 -10 -2 M ⊙ km s -1 yr -1 , depending on the assumption of the outflow inclination angle (i = 30 • -70 • ). The F out and the envelope mass derived from millimeter dust continuum observations are consistent with those of Galactic highmass protostars (e.g., Beuther et al. 2002) and the N159W-South region (TFH19). A large amount of the surrounding gas with a mass of ∼10 2 M ⊙ and the nondetection of infrared emission suggest that MMS-1 and MMS-2 are in an extremely young phase of high-mass star formation, with an age of 10 4 yr. We note that the Papillon Nebula YSO is more evolved than the two sources; however, the age is as young as ∼0.1 Myr (Paper I) judging from the combination of the spectral energy distribution (SED) fitting using the Spitzer/Herschel data and the compactness of the H i region (see also Chen et al. 2010). Although certain time differences are found, the three high-mass protostellar systems with separations of ∼1-2 pc are formed within the order of ∼0.1 Myr. We discuss the possible processes of high-mass star formation taking place in the N159E-Papillon region in Sect. 4.1. the parental molecular cloud is composed of many complex filamentary structures (Figures 1 (a) and4) rather than the previously identified several filaments (Figure 1 (b)). Therefore, it is more likely that the molecular cloud was formed in an entangled filamentary configuration rather than by a coalescence process of many individual filaments.

(2) The previous lower-resolution observations could not find the protostellar activities in MMS-1/2 (Sect. 3.3), and thus it was thought that the high-mass star-formation seen at the Papillon Nebula is a local event around this region. Despite this, the projected separations between the Papillon Nebula YSO and MMS-1/2 are as large as ∼1-2 pc, and these objects are independently formed within a few 0.1 Myr. In addition to this, our separate observations toward the N159W-South region (TFH19), which is located ∼50 pc apart from the N159E-Papillon cloud in projection, found that both regions show quite similar star-formation activities and molecular gas distributions (see also Figure 7). In N159W-South, we detected multiple protostellar sources with a separation of ∼1-2 pc, along with the massive molecular filaments (TFH19). The physical properties of the outflows and molecular filaments are quite similar to those in the N159E-Papillon region and the global orientation of both filaments is roughly parallel to each other. These facts indicate that the star formation activity and the filament formation taking place in the N159 region are not only local phenomena with a scale of 1-2 pc, but also a more global event extending up to >50 pc. We thus consider the large-scale gas kinematics as an origin of the formation of massive protostars/filaments in this region.

We calculated the filament width to be ∼0.1 pc, which is reported in a number of Galactic studies (e.g., [START_REF] André | Protostars and Planets VI[END_REF] and references therein) and its high line mass, ∼a few × 100 M ⊙ pc -1 . Such high line-mass filaments are unstable against the gravitational collapse unless some additional strong support forces emerge to regulate and slow down the collapse. There is a possibility that the massive filaments were formed by a large-scale converging flow quite recently with a timescale of ∼a few 0.1 Myr (see also Sect. 1). If we ignore the magnetic field, the virial mass per unit length, M line,vir = 2σ 2 v /G (see Fiege & Pudritz 2000), is calculated to be ∼1.4 × 10 2 M ⊙ pc -1 , as the typical velocity dispersion, σ v , is ∼0.6 km s -1 measured from the present 13 CO data. The M line,vir is smaller than the observed line mass (Sect. 3.1) and it is likely that the filaments are gravitationally unstable. Nayak et al. (2018) also reported the low virial parameters in molecular clumps in the N159 region, thus we suggest that the filaments may not be long-lived objects. Even if the strong magnetic field of ∼300 µG is penetrated perpendicular to a ∼0.1 pc width filament, a line mass of ∼70 M ⊙ pc -1 is a limit that can be magnetically supported according to Inoue et al. (2018).

We summarize the early atomic/molecular gas studies around the N159 region. Figure 7 shows the large-scale gas distributions seen in H i (Kim et al. 2003) and CO (Fukui et al. 2008;Minamidani et al. 2008, see also Wong et al. 2011). Fukui et al. (2017) subtracted the galactic rotation from the H i data and found three major velocity features, L-, I-, and D-components (see also Luks & Rohlfs 1992;Tsuge et al. 2019). The largest giant molecular complex in the LMC, called "the molecular ridge," which is shown in green contours in Figures 7(a) and (b), is mainly located at the intermediate velocity range between the L-and D-components (Fukui et al. 2017). These authors proposed that the tidal interaction between the LMC and the Small Magellanic Cloud (SMC) drove a kpc H i flow, which originated in the past close encounter between the two galaxies, which was modeled in the numerical simulations by Bekki & Chiba (2007) following the original suggestion by Fujimoto & Noguchi (1990). After settling outside the LMC disk for 200 Myr, the gas could be now inducing the formation of the super star cluster R136 in 30 Dor as well as the molecular ridge containing the N159 region (Fukui et al. 2017). Because the GMC in the N159 region is the most massive molecular reservoir of the region (e.g., Fukui et al. 2008;Minamidani et al. 2008;Chen et al. 2010), this GMC may be the end product of a cloud strongly compressed by the H i flow and enriched to the large mass we observe currently (Fukui et al. 2017). Recent MHD simulations by Inoue et al. (2018) investigated detailed processes of filament/protostar formation driven by cloud-cloud collisions. These authors demonstrated that massive hub filaments with their complex morphology that qualitatively resembles the filaments in the N159E-Papillon region, are formed in the compressed layer between the cloud and the flow, within a few × 0.1 Myr after the collisional event. The colliding flow or GMC is likely moving from the northwest to the southeast, as suggested by Fukui et al. (2017) (see also the white arrow in Figure 7 (b)), and the global orientation of the molecular filaments in the N159E/W region roughly follows the flow (Figures 7(d) and (e)). These features are reproduced by numerical simulations of colliding clouds, as demonstrated by Inoue et al. (2018). The present case in the N159E-Papillon region shows that the typical lengths of the filaments are ∼6 pc with a vertical angle of ∼90 • and they are in a north-south projected direction, as schematically shown in Figure 8 (see also Figure 4 (a)). This distribution can be explained as follows: the initial small cloud collided with the extended cloud as simulated by Inoue et al. (2018). As shown in Figures 7(a) and (b), the distribution of H i gas is inhomogeneous, and thus the colliding flow is considered to have local high-density parts. Such a kind of colliding flow can mimic a collision between a small cloud and a large one (Figure 8).

The observed filamentary cloud has a velocity gradient of ∼1.5 km s -1 pc -1 in the north-south direction (Figures 2 and 3(a) and (b)). Given the filament formation scenario based on the H i colliding flow (Figures 7 and 8, Fukui et al. 2017), we have a clear picture of the gas flow in the N159 region. Assuming that the clump interacting with the D-component is falling from the northwest (Figure 8), it is likely that the clump comprises the hub of the N159E filamentary system. We expect that the filaments are decelerated via the frictional interaction with the ambient H i gas more than the dense hub. This deceleration will cause a velocity gradient, as the hub is moving toward us, and is more blueshifted than the less dense filaments, which explains an explanation on the velocity gradient. The N159W-South also has a similar interaction with the disk H i, and shows a similar velocity gradient in the same sense (TFH19), although the observed gradient is smaller than that of the N159E filamentary system, possibly due to the projection effect. It is thus a natural outcome that the two filamentary systems have a common velocity gradient in spite of a separation of ∼50 pc.

Assuming that the ionized shock front proceeds at 5 km s -1 (e.g., Fukui et al. 2016 for RCW38), the dynamical time scale of the interaction is ∼1.2 Myr, as the size (length) of the molecular clouds is ∼6 pc. The interaction will result in the formation of an O-type star and the resulting ionization of a cloud in the following 0.1 Myrs. This means that the lower limit of the CO filament lifetime after their formation is of the order of ∼1 Myr, which is consistent with our argument that the CO filaments were formed very recently, as discussed in the previous paragraphs. The collision took place between two velocities of H i flow (Fukui et al. 2017), whereas the currently available H i data with an angular resolution of ∼1 ′ (Kim et al. 2003) are not high enough to distinguish the relation between the molecular filaments and the large-scale H i gas. The initial H i gas still needs to be observed at a resolution comparable to that of our ALMA observations to better understand its connection with the molecular reservoirs.

Comparison with Galactic Pillars of creation

It is also important to better understand how feedback is affecting the environment of high-mass stars in disentangling the gas distribution that is usually a mixture of the initial conditions prior to high-mass star formation and the results of feedback effects. In that context, we consider the formation mechanism and the evolutionary state of the CO pillars around the Papillon Nebula. In this section, we compare our study with the early results for the Eagle Nebula in M16, the most famous "pillars of creation" in the Galaxy, and compare them with the present results in N159E-Papillon. Pound (1998) observed the Eagle Nebula in 12 CO, 13 CO, and C 18 O (J = 1-0) by using the Berkeley-Illinois-Maryland Array with a resolution of ∼0.1 pc. The mean column density and volume density deduced from the CO observations are 2 × 10 22 cm -2 and ∼3 × 10 3 cm -3 , respectively. The total ionizing flux is S ∼1.2 × 10 50 s -1 , mostly from two O5 stars in M16 and the separation between the ionizing stars and the pillars is a few parsecs (Hillenbrand et al. 1993). The age of M16 is estimated to be 1.3±0.3 Myr (Bonatto et al. 2006). The Papillon Nebula, with an age of ∼0.1 Myr (Paper I) is in a much younger stage than the Eagle Nebula and the separation of the pillars is ∼0.5 pc from the YSO. Although the total ionization fluxes of both regions are almost the same or higher in the Papillon Nebula (S ≫ 1.1 × 10 50 s -1 , Paper I), the column density and volume density of the CO pillars in the Papillon Nebula, (0.9-3.0) × 10 23 cm -2 and ∼(3-7) × 10 5 cm -3 , respectively, are an order of magnitude higher than those of the Eagle Nebula. This indicates that dissociation of the molecular gas toward the CO pillars has not reached the innermost part. On the other hand, because the boundaries of the 12 CO, 13 CO, and C 18 O distributions facing the Hα emission are very close to each other (Figure 5), the surrounding low-density envelopes of the CO pillars have already been dissociated. We investigate the velocity structure of the CO pillars to explore the formation mechanism. If the Rayleigh-Taylor instability (Spitzer 1954) is working to develop pillar-like objects, the velocity gradient should increase with the square root of the distance from the top of the pillar (Frieman 1954). Pound (1998) pointed out the Rayleigh-Taylor instability is unlikely, as the formation process of the pillars in the Eagle Nebula is based on the CO velocity fields. Our observations also show similar velocity gradients to what is shown in Figure 5 of Pound (1998) or no significant velocity gradient, as shown in Figure 3. We thus exclude the Rayleigh-Taylor instability as a candidate mechanism to form the CO pillars in the Papillon Nebula. Alternatively, we suggest that the radially distributed hub-filaments were the initial configuration before the high-mass star formation (see also Figure 8). The present gas distribution of the Papillon Nebula (Figure 5) can be explained as follows: the high-mass star was formed at the intersection of the hub and the central part and the relatively low-density gases around the filaments were dissociated. Hub filaments without any ionization regions are also seen in the N159W-South clump, where ionization has not yet started (Paper I; TFH19) and it may be in an earlier stage than the Papillon Nebula. N159W-South may possibly evolve later into a stare similar to the N159E-Papillon when the protostar becomes a mature O star.

CONCLUSIONS

We carried out high-resolution observations of the N159 E region in the ALMA Cycle 4 period. Our main conclusions can be summarized belows:

1. The cloud distribution was known to be filamentary at ∼1 ′′ resolution based on the Cycle 1 observations (Paper I).

The present observations revealed that the previously identified filamentary distribution consists of even smaller filaments with a typical width of 0.1 pc at ∼0. ′′ 28 resolution in the 13 CO (J = 2-1) transition. These CO filaments appear to be extended nearly radially over 1-2 pc, with a pivot toward the Papillon Nebula, which is ionized by an O star. The line mass of the filaments is estimated to be ∼(1-7) × 10 2 M ⊙ pc -1 and is comparable to that in high-mass star-forming regions in the Milky Way.

2. One of the filaments is associated with the other two protostars driving molecular outflows with a dynamical timescale of ∼10 4 yr, which is presumably two high-mass protostars located at 1-2 pc north of the Papillon Nebula. This indicates very recent formation of at least three high-mass protostellar systems separated by ∼1-2 pc and that the star formation is coeval within ∼0.1 Myr.

3. The filamentary distributions are seen as "pillars" similar to those in M16, extending into the Papillon Nebula.

Although part of the filaments is already ionized in the H ii region, the column density of the pillars is an order of magnitude higher than that of the pillars in M16. We suggest that these pillars were formed prior to the O-star formation, and have been continuously ionized up to the present day.

4.

A recent H i study by Fukui et al. (2017) suggested that kiloparsec-scale triggering of star formation occurred about 1 Myr ago through the compression in the converging H i flows in the R136-N159 region, which was driven by the tidal interaction between the LMC and SMC. We suggest that the H i converging flows possibly triggered the coeval formation of two young star systems in the N159 region over a length scale of 50 pc encompassing both N159E and N159W. Formation of the filaments in a cloud-cloud collision is consistent with the MHD simulations of Inoue et al. (2018) which show a hub-filamentary distribution with a pivot at the O-star formation site. The present picture well explains the similar filamentary morphology of N159E and N159W, both of which show unevenly extended filaments to the north despite the large separation. The separation does not allow communication between the two nearly coeval star-forming spots within the O-star formation timescale of <1 Myr.

Figure 1 .

 1 Figure 1. 13 CO (J = 2-1) images toward the N159E-Papillon region. (a) Velocity-integrated intensity of 13 CO (J = 2-1) obtained in Cycle 4. The angular resolutions are shown in the lower left corners in each panel. The yellow crosses represent the position of the Papillon Nebula YSO, given by the 98 GHz continuum peak (Paper I). The white crosses denote the position of MMS-1 and MMS-2 (see the text in Sect. 3.3). The shape of the Papillon Nebula traced by Hα emission (see also Figure 5 (b)) is shown in white contours in both panels. (b) Same as (a) but for the 13 CO (J = 2-1) data obtained in Cycle 1 (Paper I). Note that the moment masked cube data are used in both panels (see Sect. 2).

Figure 2 .

 2 Figure2. Velocity-channel maps toward the N159E-Papillon region in 13 CO (J = 2-1). The lowest velocities are given in the upper left corners in each panel. The white/yellow crosses and white lines are the same as those in Figure1. The angular resolution is given by a white ellipse in the lower left corner in the lower left panel.

Figure 3 .

 3 Figure 3. Velocity structures of filamentary molecular clouds traced by the CO observations in the N159E-Papillon region. (a) The first-moment intensity-weighted velocity map of 13 CO (J = 2-1) is shown in color scale. Note that the moment-masked cube data is used (see Section 2). The white/yellow crosses are the same as those in Figure 1. The angular resolution is shown in the lower left corner. (b) Averaged spectra of 12 CO, 13 CO, and C 18 O (J= 2-1) in the regions where the molecular line emissions were detected. (c) A 13 CO (J = 2-1) position-velocity diagram along the region shown by dashed rectangle in panel (a). The X-axis shows angular distances in parsec scale from the position of MMS-1. The dotted line shows the velocity gradient (1.5 km s -1 pc -1 ) derived by the least-squares fitting (see the text).

Figure 4 .

 4 Figure 4. Filamentary structures and their width toward the N159E-Papillon region. (a) Color lines show the identified filaments (see the text) on the 13 CO velocity-integrated intensity map. (b) The intensity profile of filament 1. The grey dots are all the points of the cross sections taken along the filament. The blue and red lines show the averaged profile and the Gaussian fitting result for the profile, respectively. (c) Distributions of the FWHM widths for identified filaments. (d) The number of profiles (every radial slice) along all the identified filaments.

Figure 5

 5 Figure5shows the 12 CO (panels (a)-(c)), 13 CO, C 18 O (J = 2-1) (panel (d)) and the Hα (panel (b)) distributions around the Papillon Nebula. Our previous observations confirmed that there is a CO cavity centered on the Papillon Nebula YSO (Paper I). The present high-resolution data clearly resolved several elongated structures along the Hα bright region, the Papillon Nebula, as shown in Figure5(b). These structures roughly extend toward the position of the Papillon Nebula YSO and are considered to be heated by the protostars and the H ii region because they show high-brightness temperature (60-80 K) in 12 CO (Figure5(c)). We define these structures as "CO pillars" by eye based on the abovementioned features, independently of the filament identification (Sect. 3.1), and the CO pillars are shown by white lines in Figure5 (c). This is the first detection of "pillars of creation" in an external galaxy (seeHester et al. 1996 for the Eagle Nebula). The properties of the CO pillars are listed in Table1. The averaged column densities of the CO pillars are (0.9-3.0) × 10 23 cm -2 . We derived the width (FWHM) of the pillars by Gaussian fitting to the averaged column density profile perpendicular to the white lines in Figure5. The resultant width of the pillars is ∼0.1-0.2 pc, which is similar to that of the filaments in the region. The average volume density, n(H 2 ), is ∼(1-7) × 10 5 cm -3 . These results show that the density and temperature are significantly higher than those of the surrounding filaments. The high-density nature is also supported by the detection of the C 18 O (J = 2-1) emission along the CO pillars (Figure5 (d)). We do not see clear extinctions of the Hα emission along the CO pillars, except for the central part of the Papillon Nebula, despite its high-column density. This indicates that the pillars are embedded in the ionized region or located behind it.

  Figure 5. Zoomed-in views of CO, its isotopes, and Hα emission in the Papillon Nebula. (a) Total velocity-integrated intensities of 12 CO (J = 2-1) are shown in color scale. (b) Color scale and contours of the Hα emission obtained with HST and the 12 CO integrated intensity as in panel (a), respectively. The CO contour levels are 50, 100, 250, and 300 K km s -1 . (c) Color-scale image of the peak brightness temperature map of 12 CO (J = 2-1). (d) Color-scale image and contours of total velocity-integrated intensities of 13 CO (J = 2-1) and C 18 O (J = 2-1), respectively. The C 18 O contour levels are 0.5, 1.5, 2.5, and 3.5 K km s -1 . The yellow crosses in all panels represents the position of the Papillon Nebula YSO. The angular resolutions of the ALMA data are shown in the lower left corners in each panel.

  Heydari-Malayeri et al. (1999);[START_REF] Testor | [END_REF] for the 21 M⊙ protostar, and Indebetouw et al. (2004); Chen et al. (2010) for the 41 M⊙ one.b The average total luminosity; seeCarlson et al. (2012) andChen et al. (2010) for details.

Figures

  Figures 6(d) and (e)).

  Filament/Protostar formation in the N159E-Papillon regionWe discuss the formation of the protostars and the filaments in the N159E-Papillon region. In Paper I, we discussed the possibility that collisions by three molecular filaments induced the star formation activity in the Papillon Nebula. However, such a filamentary cloud collision model may not be appropriate to explain the star formation activities in this region revealed by the present observations for some reasons. (1) The present observations clearly show that

Figure 6 .

 6 Figure 6. Distributions of high-velocity outflow wings and filamentary structures traced by continuum/line emissions toward the N159E-Papillon region. (a) Color-scale image and lilac contours of 1.3 mm continuum and velocity-integrated intensities of C 18 O (J = 2-1) emission, respectively. The contour levels are 0.5, 1.5, 2.5, and 3.5 K km s -1 . The white/yellow crosses and the white contour are the same as those in Figure 1. (b) and (c) Enlarged views of 1.3 mm continuum and outflow wings toward MMS-1 and 2. The grayscale images show the 1.3 mm continuum. The blue and red contours show images of 12 CO (J = 2-1) integrated over the velocity ranges shown by red and blue dashed lines in panels (d) and (e). The lowest contour and the subsequent contour step are 10 and 5 K km s -1 , respectively. (d) and (e) Black lines show the averaged intensity profiles of 12 CO (J = 2-1) toward the 1.3 mm continuum peaks of MMS-1 and MMS-2. The gray lines show the reference 12 CO spectra at the positions of ∼1 ′′ away from the millimeter sources. The black dotted lines show the systemic velocities estimated from Gaussian fitting to the 13 CO profile at each continuum peak.

Figure 7 .

 7 Figure 7. Large-and small-scale distributions of H i and CO emission around the N159 GMC. (a) The position-velocity diagrams of H i (Kim et al. 2003) and CO (J = 1-0) (Fukui et al. 2008) are shown in grayscale and green contours, respectively. The extracted ranges to make the maps are shown as white dashed lines in panel (b). (b) Cyan and red images of H i integrated intensity maps of L-(V offset = -100 to -30 km s -1 ), and the D-component (V offset = -10-10 km s -1 ), respectively, reproduced from Fukui et al. (2017), and Tsuge et al. (2019). The green contours show the integrated intensity map of CO (J = 1-0) with an angular resolution of 2. ′ 7. (c) The green image and contours show the CO (J = 3-2) map of the N159 GMC (Minamidani et al. 2008). (d) and (e) ALMA 13 CO images of the N159E-Papillon (same as Figure 1 (a)) and the N159W-South regions (TFH19). The shape of the Papillon Nebula traced by Hα emission is shown in magenta contours in panel (d). The positions of the protostellar sources, the Papillon Nebula YSO, and outflow sources with and without 1.3 mm continuum sources are denoted by the yellow, black, and white crosses, respectively.

Figure 8 .

 8 Figure 8. Schematic views of the collisional event in the N159E-Papillon region. A large-scale view of the colliding event with the size of more than 10 pc is shown in the left panel. The zoom on views in the middle and right panels show what is happening around the shock front. Black and gray colors represent the CO distributions traced by the ALMA observations and expected H i gas distributions, respectively.

Figure 9 .

 9 Figure 9. Flux comparison between the Cycle 4 and Cycle 1 data in the N159E-Papillon region. (a) The smoothed velocityintegrated intensity map of the Cycle 4 13 CO (J = 2-1) data with an angular resolution of 1. ′′ 3 × 0. ′′ 87 (the Cycle 1 data). The angular resolution is shown by ellipses in the lower left corners in panels (a) and (b). (b) The distributions of the Cycle 4/Cycle 1 intensity ratio. (c) The histogram of the Cycle 4/Cycle 1 intensity ratio shown in panel (b).

Table 1 .

 1 Properties of CO pillars around the Papillon Nebula N (H2)max (cm -2 ) b N (H2)ave (cm -2 ) c Width (pc) d n(H2)ave (cm -3 ) e

	Name (K) Pillar 1 T 12 CO max 77	4.0 × 10 23	2.5 × 10 23	0.12	6.7 × 10 5
	Pillar 2	76	3.7 × 10 23	2.6 × 10 23	0.18	4.7 × 10 5
	Pillar 3	80	4.1 × 10 23	3.0 × 10 23	0.22	4.4 × 10 5
	Pillar 4	67	1.3 × 10 23	1.0 × 10 23	0.13	2.7 × 10 5
	Pillar 5	61	1.4 × 10 23	9.0 × 10 22	0.09	3.2 × 10 5

a

Table 2 .

 2 YSO characteristics and outflow properties in the N159E-Papillon region Mout (M⊙) c Distance (pc) d Velocity (km s -1 ) e

	Source name	M Blue/Red	Blue/Red	Blue/Red
	Papillon Nebula YSO	21 + 41	(0.5-1.8) × 10 5

* (M⊙) a L * (L⊙) b

The previous observations in the Cycle 1 with an angular resolution of ∼1 ′′ (∼0.24 pc) did not resolve the thin filaments; however, the Cycle 4 data clearly identified much narrower filaments that were previously unresolved. To identify the filamentary structures from the 13 CO data quantitatively, we applied the DisPerSE algorithm(Sousbie 2011), which has been developed to extract structures such as filaments and voids in astrophysical data. This algorithm was applied to the dust continuum data obtained by the Herschel Gould Belt survey (e.g.,André et al. 2010;Arzoumanian et al. 2011) and molecular line emission in Taurus(Panopoulou et al. 2014). To extract topological components from the molecular line cube, we set a "persistence" threshold, and the difference in value between the two points in the pair lower than the given threshold is cancelled (see Sousbie 2011), of 20 K in brightness temperature of 13 CO (J = 2-1), which roughly corresponds to the typical peak temperature in the high-column density ( 5 × 10 22 cm -2 ) regions. We defined structures with an aspect ratio >3:1 as the filaments after the measurements of the width (see below) and length. For this reason, some of the pillars (Sect. 3.2) and high-column density regions around the Papillon Nebula are not identified in this criterion. In total, we identified ∼50 filaments that are overlaid on the 13 CO velocity-integrated intensity map in Figure4(a). To estimate filamentary widths, we performed Gaussian fits to the

We thank Doris Arzoumanian and Shu-ichiro Inutsuka for discussions about the filamentary molecular clouds. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2012.1.00554.S, and #2016.1.01173.S. ALMA is a partnership of the ESO, NSF, NINS, NRC, NSC, and ASIAA. The Joint ALMA Observatory is operated by the ESO, AUI/NRAO, and NAOJ. This work was supported by NAOJ ALMA Scientific Research grant No. 2016-03B and JSPS KAKENHI (grants No. 22244014, 23403001, 26247026, 18K13582, 18K13580, and 18H05440). The work of M.S. was supported by NASA under award number 80GSFC17M0002.