J Larsson

C Fransson

G Östlin

P Gröningsson

A Jerkstrand

C Kozma

J Sollerman

P Challis

R P Kirshner

R A Chevalier

K Heng

R Mccray

N B Suntzeff

P Bouchet

A Crotts

J Danziger

E Dwek

K France

P M Garnavich

S S Lawrence

B Leibundgut

P Lundqvist

N Panagia

C S J Pun

N Smith

G Sonneborn

L Wang

J C Wheeler

X-ray illumination of the ejecta of Supernova 1987A

When a massive star explodes as a supernova, substantial amounts of radioactive elementsprimarily 56 Ni, 57 Ni and 44 Ti-are produced 1 . After the initial flash of light from shock heating, the fading light emitted by the supernova is due to the decay of these elements 2 . However, after decades, the energy for a supernova remnant comes from the shock interaction between the ejecta and the surrounding medium 3 . The transition to this phase has hitherto not been

observed: supernovae occur too infrequently in the Milky Way to provide a young example, and extragalactic supernovae are generally too faint and too small. Here we report observations that show this transition in the supernova SN 1987A in the Large Magellanic Cloud.

From 1994 to 2001 the ejecta faded owing to radioactive decay of 44 Ti as predicted. Then the flux started to increase, more than doubling by the end of 2009. We show that this increase is the result of heat deposited by X-rays produced as the ejecta interacts with the surrounding material. In time, the X-rays will penetrate farther into the ejecta, enabling us to analyse the structure and chemistry of the vanished star.

Due to the proximity of SN 1987A (located only 160, 000 light years away), we can study the evolution of the supernova (SN) in great detail. The central ejecta are surrounded by a ring of circumstellar material (Fig. 1) that was shed from the star 20, 000 years before the explosion in 1987 [START_REF] Morris | The Triple-Ring Nebula Around SN 1987A: Fingerprint of a Binary Merger[END_REF] . Since the explosion, the ejecta have been expanding, and now the outer parts of the ejecta are colliding with the ring, causing it to brighten at all wavelengths [START_REF] Gröningsson | Time evolution of the line emission from the inner circumstellar ring of SN 1987A and its hot spots[END_REF][START_REF] Racusin | X-ray Evolution of SNR 1987A: The Radial Expansion[END_REF][START_REF] Zanardo | Multifrequency Radio Measurements of Supernova 1987A Over 22 Years[END_REF][START_REF] Dwek | Five Years of Mid-infrared Evolution of the Remnant of SN 1987A: The Encounter Between the Blast Wave and the Dusty Equatorial Ring[END_REF] . The dense, central part of the ejecta contains most of the mass from the disrupted star and acts as a calorimeter for the energy input to the SN. We have determined the energy input by tracking the energy output with the Hubble Space Telescope (HST).

Because the ejecta are roughly elliptical in projection on the sky we use an elliptical aperture to measure the brightness. To monitor a constant mass of expanding material, we allowed the measuring aperture to expand linearly with time. The axes of the aperture were therefore three times larger in 2009 than in 1994 (Fig. 1). Using this aperture, we determined the R-and and B-band light curves of the ejecta, as shown in Fig. 2 (see Supplementary Table 1 and Supplementary Information, section 1 for further details of the observations and light curves). Our measurements show that the flux from the ejecta decays during the first ∼ 5, 000 days after the explosion, as expected from radioactive input, but then starts to increase, reaching a level that is 2-3 times higher around day 8, 000 (end of 2009). A new energy source must be present in addition to radioactive decay. Below, we consider a model for the declining phase and then discuss the new energy source that is responsible for the observed increase in flux.

The energy input to the declining phase of the light curve after ∼ 1, 500 days is expected to come from positrons produced in the decay of 44 Ti 2, 9, 10 . To test this we use a model 2 with abundances taken from the 14E1 explosion model [START_REF] Shigeyama | Theoretical light curve of SN 1987A and mixing of hydrogen and nickel in the ejecta[END_REF] and a 44 Ti mass of 1.4 × 10 -4 M ⊙ [START_REF] Jerkstrand | The 44 Ti-powered spectrum of SN 1987A. Astron[END_REF] (Supplementary Information, section 3). The model is shown in Fig. 3 together with the observed broadband luminosities. The good agreement with the observations up to day 5, 000 confirms that the 44 Ti positrons provide the energy input up to this point. However, after day 5, 000 the model clearly fails to describe the light curve; radioactive decay cannot explain the increase in flux that we observe.

One possible origin for the flux increase is the reverse shock that results from the interaction between the ejecta and the H II region inside the ring [START_REF] Michael | Hubble Space Telescope Observations of High-Velocity Lyα and Hα Emission from Supernova Remnant 1987A: The Structure and Development of the Reverse Shock[END_REF][START_REF] Smith | The Reverse Shock of SNR 1987A at 18 Years after Outburst[END_REF][START_REF] Heng | Evolution of the Reverse Shock Emission from SNR 1987A[END_REF][START_REF] France | Observing Supernova 1987A with the Refurbished Hubble Space Telescope[END_REF] . The reverse shock produces strong Lyα and Hα emission, which increased by a factor of ∼ 1.7 between 2004 and 2010 [START_REF] France | Observing Supernova 1987A with the Refurbished Hubble Space Telescope[END_REF] . Although most of this emission originates close to the ring, there is also a component of projected high-velocity Hα-emission that can be traced to the central parts of the ejecta [START_REF] France | Observing Supernova 1987A with the Refurbished Hubble Space Telescope[END_REF] and which would therefore contribute to the flux we measure. To determine the contribution of the reverse shock to our light curves we have examined HST STIS spectra from 2004 and 2010 (Supplementary Information, section 2 and Supplementary Fig. 5). The reverse shock can be isolated in the spectra because of its boxy line profile, allowing us to place a limit on its contribution at ∼ < 20%. Furthermore, this changes only marginally between 2004 and 2010, as the expanding measuring aperture remains well inside the area where most of the shock emission is seen. Importantly, an increase in flux is also seen in the [Ca II] doublet lines at rest wavelengths 7,292 Å and 7,324 Å between 2000 -2010 (determined from UVES observations at the ESO/VLT, Fig. 2). These lines have speeds of ∼ < 5, 000 km s -1 , implying that they originate in the inner ejecta (the projected ejecta speed near the edge of the ring is ∼ > 7, 000 km s -1 at the present time). We conclude that the increase in flux occurs primarily in the inner ejecta and cannot be explained by emission from the shock region.

We believe that the strong X-ray flux produced in the ring collision is the dominant source of energy input to the ejecta. The X-ray flux from the ring increased by a factor ∼ 3 in the 0.5 -10 keV band between day 6, 000 and day 8, 000 [START_REF] Racusin | X-ray Evolution of SNR 1987A: The Radial Expansion[END_REF] , similar to what we find for the optical emission from the ejecta. To investigate this, we calculated the fraction of X-rays absorbed by the ejecta from a point source located at the ring, using the partially mixed 14E1 explosion model [START_REF] Blinnikov | Radiation Hydrodynamics of SN 1987A. I. Global Analysis of the Light Curve for the First 4 Months[END_REF] . As shown in Supplementary Fig. 6, most of the observed X-ray flux is absorbed in the core region of the ejecta (corresponding to speeds less than 5, 000 km s -1 ), where most of the heavy elements reside. At an energy of ∼ 0.35 keV, which corresponds to the temperature of the dominant component in the X-ray spectrum [START_REF] Zhekov | Evolution of the Chandra CCD spectra of SNR 1987A: probing the reflected-shock picture[END_REF] , the fraction of flux absorbed by the ejecta at t yr years can be approximated by 1.6 × 10 -3 t 1.67 yr (the increase with time is mainly due to the increasing solid angle of the expanding ejecta, assumed to be spherical, as seen from the ring). This gives a present-day absorbed Xray luminosity of ∼ 5.0 × 10 35 erg s -1 . In this calculation we have neglected the weaker, hard component that contributes to the X-ray spectrum [START_REF] Zhekov | Evolution of the Chandra CCD spectra of SNR 1987A: probing the reflected-shock picture[END_REF] . We note that this does not significantly affect the estimate of the absorbed flux, although the hard X-rays may be important due to their larger penetrating power.

To model the ejecta light curve produced by input from the X-rays we scaled the observed X-ray flux 6 by the fraction absorbed at 0.35 keV, multiplied the resulting flux with a constant (corresponding to the conversion efficiency from X-rays to optical) and added this to the radioactive energy input. Fig. 3 shows the scaled X-ray flux together with the observed light curves. This model follows the general trend of the observed fluxes in both bands, although we note that a more accurate model would need to take into account the detailed shape of the X-ray spectrum and the reprocessing of the X-rays into optical emission. The required conversion efficiency from X-rays to optical emission in our model is 5.0% in the R-band and 3.1% in the B-band.

The conversion of X-rays to optical/IR emission is similar to that of the 44 Ti positrons. Both involve degradation of non-thermal electrons to heating, ionization and excitation. For a typical ionization fraction of 10 -3 -10 -2 , the expected efficiency of conversion from X-rays to Hα (the dominant line in the R-band) is ∼ 5% (Supplementary Information, section 3). This conversion factor is consistent with the scaling factor we used to model the light curve. Similar arguments apply to the B-band. Furthermore, the density in the core is high enough for the time-scale of recombination to be shorter than the expansion time-scale, ensuring a balance between the energy input and output.

Other possible explanations for the increase in flux include input from a central pulsar [START_REF] Woosley | Hard emission at late times from SN 1987A[END_REF] , a transition from optically thick to optically thin dust and positron leakage from the Fe-rich regions.

We find that input from a pulsar is unlikely for several reasons. In particular, it would be a strange coincidence for the emission from the pulsar to mimic the increasing X-ray flux from the ring interaction. Also, we expect the energy input from a pulsar to be concentrated toward the lowvelocity material at the centre of the ejecta, but observations of the Hα and [Ca II] lines show that the increase occurs for speeds up to ∼ 5, 000 km s -1 . We also note that constraints on a point source at the centre of SN 1987A have already been obtained using HST data taken near the minimum of the ejecta light curve [START_REF] Graves | Limits from the Hubble Space Telescope on a Point Source in SN 1987A[END_REF] . A change in the properties of the dust or a transition in the positron deposition process are also unable to quantitatively explain the observed increase in flux (Supplementary Information, section 4).

We conclude that SN 1987A has made a transition from a radioactively dominated phase to a phase dominated by the X-ray input from the ring collision. This conclusion has interesting implications for the observed morphology. In particular, most of the X-rays are likely to be absorbed at the boundary of the ejecta core, where the density increases rapidly. This may lead to the light from the ejecta being emitted in a ring that is concentrated to the plane of the circumstellar ring. The 'hole' in the ejecta (Fig. 1), which has become more pronounced since ∼ 2001, may in fact be a result of this, rather than reflecting the true density distribution or dust obscuration. The asymmetric morphology seen at speeds of ∼ < 3, 000 km s -1 in the near-IR [Si I] and [Fe II] lines [START_REF] Kjaer | The 3-D structure of SN 1987A's inner ejecta[END_REF] is, however, likely to be intrinsic to the metal core. By studying future changes in the morphology of the ejecta, we will be able to understand the origin of this asymmetry

In the future the density of the ejecta will decrease and the fraction of X-rays absorbed will grow (Supplementary Fig. 7). As a result the ionization will increase and a smaller fraction of the X-ray flux will produce line excitation. A larger fraction will go into heating, leading to an increase in the mid-IR flux and a flattening of the optical light curves. In time, the X-rays will also penetrate deeper layers of the ejecta, thereby allowing us to probe the chemical structure of the innermost ejecta. This will be a novel form of X-ray tomography. ellipse shows the fixed aperture used for one of the light curves in Supplementary Fig. 2. The R-band emission from the ejecta is dominated by Hα emission with a small contribution from [Ca I-II] lines, whereas the B-band (Supplementary Fig. 1) is dominated by H I and Fe I-II lines [START_REF] Jerkstrand | The 44 Ti-powered spectrum of SN 1987A. Astron[END_REF][START_REF] Chugai | Hubble Space Telescope Spectrum of SN 1987A at an Age of 8 Years: Radioactive Luminescence of Cool Gas[END_REF] .

Only the densest, central parts of the ejecta are visible due to the low surface brightness of the outer parts. In reality, the ejecta extend to the ring, as is evident from the strong interaction with the ring. to have a 50% covering factor of the core inside 2000 km s -1 ) [START_REF] Lucy | Dust Condensation in the Ejecta of Supernova 1987A -Part Two[END_REF][START_REF] Wooden | Airborne spectrophotometry of SN 1987A from 1.7 to 12.6 microns -Time history of the dust continuum and line emission[END_REF] . The lower dot-dashed lines show the light curves with no 44 Ti, illustrating the necessity to include this isotope. The red dashed lines show a model based on a constant fraction of the observed X-ray flux [START_REF] Racusin | X-ray Evolution of SNR 1987A: The Radial Expansion[END_REF] , corrected for the fraction of flux absorbed by the ejecta (Supplementary Fig. 7). The blue solid line shows the sum of both models. The R-and B-bands contain 5% and 3% of the total bolometric flux, respectively, and we expect these fractions to remain roughly constant with time. This is because the relative amount of energy resulting in heating, ionization and excitation will remain nearly constant as long as the ionization by the X-rays is ∼ < 10 -2 . It is clear from this figure that there is a transition at ∼ 5, 000 days from a radioactivity dominated phase to a phase dominated by the X-ray input from the collision with the ring.

to measure the evolution of the flux. The shape of the ellipse was determined by fitting elliptical isophotes to the ejecta region in the observation from 2000-11-13. This epoch was chosen because it is long enough after the explosion that the ejecta shape is clearly discernible, while still early enough for the edge of the inner ejecta to be clearly separated from the circumstellar ring. The fits were performed using the IRAF/STSDAS task ELLIPSE. Setting the semi-major axis (sma) to 0.2 ′′ (the size used for the light curve in Fig. 2 for this epoch) we find a position angle of 17.4 ± 0.1

•
and an eccentricity of 0.684 ± 0.002. This can be compared with previous measurements with HST (P.A. = 14 ± 5 • ) [START_REF] Wang | The Axisymmetric Ejecta of Supernova 1987A[END_REF] and VLT/SINFONI (P.A. = 15 ± 0.9 • , e = 0.83, from the 1.644 µm Si I/Fe II feature) [START_REF] Kjaer | The 3-D structure of SN 1987A's inner ejecta[END_REF] . The agreement with the position angle is clearly good and, given that the SINFONI measurements probe the Si/Fe core and the R-band is dominated by the H-rich gas, the agreement with the eccentricity is also reasonable. As described in the main paper, the light curves presented in Fig. 2 were derived using this elliptical aperture. The size of the aperture was expanded as a function of time in order to always follow the same ejecta material.

In order to make sure that the transition from a decaying to a rising flux seen in Fig. 2 is not connected to the use of this particular aperture, we also performed the measurements using apertures with other shapes and sizes, trying both apertures that have a fixed size and apertures that expand with the ejecta. Supplementary Fig. 2 shows the R-and B-band light curves for the same expanding elliptical aperture as in Fig. 2, an expanding circular aperture (with radius equal to the sma of the elliptical aperture) as well as a large, fixed aperture. The fixed aperture encloses most of the ejecta but still avoids direct contamination by the ring (the aperture is shown in Fig. 1).

The light curves extracted from the expanding elliptical and circular apertures show very similar behaviour, demonstrating that the exact shape of the aperture is not important. The light curve from the large, fixed aperture also shows the same behaviour during the first ∼ 7,000 days, but levels off at late times. This is at least partly due to ejecta material expanding out of the fixed aperture. The large aperture is also likely to be significantly affected by contamination from the circumstellar ring, making it hard to draw firm conclusions regarding the ejecta using this aperture.

Contamination from the ring (due to flux from the brightening ring spilling into the ejecta region) is likely to affect all our light curves at some level. In order to quantify this effect we used the HST point-spread-function modelling tool Tiny Tim [START_REF]Tiny Tim HST PSF Modeling[END_REF] and created models for the ring for a number of different epochs in both bands. We set up the models to match the observed distribution of hot spots as closely as possible and to have the same total flux in the ring area as the observations. The modelling is illustrated in Supplementary Fig. 3, which shows the real data and the model for the F675W image from 2009-04-29, and in Supplementary Fig. 4, which compares the radial profiles of the data and the model. The corrected light curves derived using these models

are shown as open symbols in Fig. 2 and Supplementary Fig. 2. The contribution from the ring increases with time and is larger in the R-band, but it does not have a major effect on the light curves from the expanding apertures. For the large, fixed aperture the contamination is much more severe, reaching about 50% of the total flux at late times in the R-band.

The modelling of the ring is naturally associated with some uncertainties. In order to quantify these uncertainties we performed two different tests. First, we investigated the effect of varying the number of points sources used to model the ring. Specifically, we created models using between 8 and 90 point sources distributed along the ring, while always keeping the total flux in the ring area constant. The different ring models resulted in fluxes in the ejecta apertures that agreed to within 15%. This demonstrates that the specific model for the ring is not very important as long as the same total flux is distributed in an ellipse around the ejecta. The main reason for this is of course that we are interested in the total flux that leaks into the ejecta region and not its detailed distribution.

Another test of the models is to compare the flux in the synthetic and real images in regions located just outside the circumstellar ring. We used clean regions (uncontaminated by stars and other sources of emission) that allow us to measure the flux in the wings of the point sources in the ring. For the observation from 2009-04-29 we find that the agreement is excellent (the discrepancy is less than 10%), which gives us confidence that the models provide a good estimate of the emission from the ring. For earlier epochs it is harder to evaluate the models in this way since the clean regions outside the ring are completely dominated by the sky background rather than ring emission. For this reason we cannot determine whether our models are equally accurate at early times. We note, however, that the flux from the ring is negligible at these times.

An additional source of uncertainty is the loss of Charge Transfer Efficiency (CTE). This effect is known to increase with time and is especially important for faint objects on low backgrounds located far from the detector readout [START_REF] Dolphin | A Revised Characterization of the WFPC2 CTE Loss[END_REF] . To investigate the importance of CTE losses we performed aperture photometry on about 10 stars of varying brightness located around SN 1987A.

We found that the effect is negligible in the ACS but clearly noticeable in WFPC2, where stars lose at most ∼ 10% of their flux between 1994 and 2009. This also agrees with estimates obtained for the supernova ejecta using the WFPC2 CTE correction formula [START_REF] Dolphin | A Revised Characterization of the WFPC2 CTE Loss[END_REF] . In terms of the light curve in Fig. 2, we note that a ∼ 10% increase in the WFPC2 data points at late times would improve the agreement between the ACS and WFPC2.

Spectroscopy

As a complement to the photometry we investigate HST STIS spectra taken with the G750L grating in 2004 and 2010. These spectra overlap with the wavelength region covered by the R-filters and

give us an opportunity to study the different spectral components that may contribute to the increase in flux. In both observations a slit with a width of 0.2 ′′ was placed in the north-south direction, centred on the ejecta [START_REF] France | Observing Supernova 1987A with the Refurbished Hubble Space Telescope[END_REF][START_REF] Heng | Balmer-dominated Shocks Revisited[END_REF] . In 2004 there were also two observations taken with the narrow slit placed on either side of the central slit. To allow for a comparison with the photometry we extracted spectra in regions that match the size of the expanding elliptical aperture as closely as possible.

This means using the central slit only and an extraction height of ±0.24 ′′ in 2004 and ±0.31 ′′ in 2010.

Supplementary Fig. 5 shows the Hα profiles from 2004 and 2010. The line profiles are clearly asymmetric with a strong blue wing extending down to ∼ -5, 000 km s -1 and a red wing that only reaches velocities of about ∼ 2, 500 km s -1 . The constant width of the ejecta line profile

shows that the expanding aperture that we used follows the same material over a 6-year span of time. From the two-dimensional spectra [START_REF] France | Observing Supernova 1987A with the Refurbished Hubble Space Telescope[END_REF][START_REF] Heng | Balmer-dominated Shocks Revisited[END_REF] it is clear that the blue wing mainly comes from the northern part of the ejecta. The lack of a correspondingly strong red wing may be due to dust absorption in the far side of the ejecta. Supplementary Fig. 5 also shows that the increase in flux between 2004 and 2010 is mainly due to an increase in the blue wing of the ejecta profile.

The Hα emission also contains a small contribution from the reverse shock. This component is clearly visible on the red side of the line as the boxy part of the profile between ∼ 3, 000 -9, 000 km s -1 . On the blue side of the line the reverse shock appears at velocities higher than ∼ -9, 000 km s -1 , where it is blended with the broad [O I] λλ 6300, 6364 lines from the ejecta at -12, 022 km s -1 and -9, 096 km s -1 . To estimate the flux from the reverse shock we focus on the boxy part of the profile on the red side, since this is not blended with any other strong lines. The contribution from this component compared to the total flux in the R-band is ∼ 9 -10% in both 2004 and 2010. It is clear from inspection of the spectra that the reverse shock is not as strong on the blue side, and a very conservative upper limit on the total contribution from the reverse shock is therefore ∼ 20%. We especially note that there is no significant increase between 2004 and 2010.

In Supplementary Fig. 5 we also show the spectrum obtained by combining the three slit positions from the 2004 observations, renormalized to the continuum of the other spectra. The combined spectrum is very similar to that from the central slit, albeit with a slightly stronger line core and red wing. The contribution from the reverse shock on the red side is ∼ 11%, i.e. only marginally larger than the values given above.

The STIS observation from 2010 also allows us to investigate the emission from the south-ernmost part of the ejecta, which is seen outside both the expanding and the fixed aperture at late times (Fig. 1). STIS spectra extracted in this region show a significant contribution from the reverse shock but also an asymmetric line profile from the ejecta, with a strong blue wing extending down to ∼ -2, 000 km s -1 . The fact that we see blueshifted emission from the southern part of the ejecta shows that there must be some ejecta material located well above the plane of the ring.

The [Ca II] observations in Fig. 2 were performed between 2000 and 2009 with the Very Large Telescope (VLT) at ESO, using the UVES instrument. The primary purpose of these observations was to follow the evolution of the narrow lines from the ring collision. To maximise the spectral resolution and to minimise contamination from nearby stars a comparatively narrow slit, 0.8 ′′ wide, was chosen. The seeing was in most cases 0.6 -0.8 ′′ , which is just enough to spatially resolve the ring into a northern and a southern part. The slit was centred on the position of the supernova and had a position angle of 30 • in all the observations. The observations were reduced with the standard pipeline and the errors on the absolute fluxes were estimated to be less than 20 -30% (ref. 5).

Modelling of the light curve

At first, the energy radiated by a SN comes from the shock wave arriving at the surface of the exploding star. This phase lasted lasted only a few hours in SN 1987A, and was followed by a period of a few weeks where the light curve was powered by the diffusive release of the internal energy produced by the shock wave [START_REF] Blinnikov | Radiation Hydrodynamics of SN 1987A. I. Global Analysis of the Light Curve for the First 4 Months[END_REF] . Subsequently, the energy for emission was supplied by the decay of radioactive isotopes created in the explosion. During the first ∼ 1, 000 days the radioactive input was dominated by 56 Ni, decaying into 56 Co and 56 Fe. From the first two years of the bolometric light curve the mass of 56 Ni could be determined to 0.069 M ⊙ 31 . After ∼ 1, 000 days 57 Ni took over and a mass of 3×10 -3 M ⊙ was found [START_REF] Kurfess | Oriented Scintillation Spectrometer Experiment observations of Co-57 in SN 1987A[END_REF][START_REF] Varani | Direct Observation of Radioactive Cobalt Decay in Supernova 1987A[END_REF][START_REF] Fransson | The freeze-out phase of SN 1987A -Implications for the light curve[END_REF] . Finally, after ∼ 1, 600 days the next most abundant isotope, 44 Ti, has been predicted to dominate the energy input to the ejecta. While the bolometric light curve could be accurately determined up to ∼ 1, 000 days, the dominance of the mid-IR, which is difficult to determine from observations, requires accurate modelling of the optical bands at later times. In addition, the hydrogen-rich parts of the ejecta are subject to a freezeout [START_REF] Fransson | The freeze-out phase of SN 1987A -Implications for the light curve[END_REF] , where recombination and cooling become slow compared to the radioactive decay timescales and the expansion time-scales. It is therefore necessary to use a time-dependent approach for the modelling.

The modelling of the radioactively powered part of the light curves in Fig. 3 is based on a time-dependent spectral code [START_REF] Kozma | Late Spectral Evolution of SN 1987A. I. Temperature and Ionization[END_REF][START_REF] Kozma | Late Spectral Evolution of SN[END_REF] . The code includes calculations of the non-thermal excitation and ionization by the gamma-rays and positrons from the radioactive decay of 56 Ni, 57 Ni and 44 Ti, as well as a full non-local thermodynamic equilibrium treatment of the most important elements and lines. The present version also includes updated atomic data, in particular more accurate collision rates for Fe II, and other ions. The radiative transfer uses the Sobolev approximation, which treats the line escape in individual lines accurately. However, the non-local scattering from line to line is not included (except for the Ca II H and K lines, which feed the [Ca II] λλ7291.5, 7323.9 lines by fluorescence). This especially affects the UV and blue parts of the spectrum and different rays in the ejecta from a point source located at the radius of the circumstellar ring. For the abundance and density distributions we used the partially mixed 14E1 explosion model [START_REF] Blinnikov | Radiation Hydrodynamics of SN 1987A. I. Global Analysis of the Light Curve for the First 4 Months[END_REF] . This model is spherically symmetric (although mixed). Unfortunately, no 2-D or 3-D simulations exist with appropriate nucleosynthesis. The spherical approximation should, however, be sufficient for the qualitative nature of the model. Most of the envelope is transparent to X-rays with energies above about 0.2 keV as seen in Supplementary Fig. 6, which shows the ejecta expansion velocity for optical depth τ = 1 as a function of photon energy. The high metallicity core dominates the opacity by K-and L-shell absorption by oxygen and other elements, and is opaque at all but the very highest energies.

The energy deposition is an angular average of the absorbed intensity over all directions of the core and envelope, shown in Supplementary Fig. 7 for four different photon energies.

The different deposition curves can be well approximated by C(E) × 10 -3 t 1.67 yr , where C(E) = (4.5, 1.6, 0.7) × 10 -3 for E = 0.1, 0.35, and 1 keV, respectively. The general time dependence can be explained as a balance between the increase of the solid angle of the core as seen from the ring, ∝ t 2 , and the increasing transparency of the ejecta as the density decreases.

The X-ray luminosity used in this paper is that directly observed by Chandra 6 . This is likely to be an underestimate for several reasons. The quoted X-ray luminosities only refer to the observed flux ∼ > 0.5 keV. For a shock with a temperature of 0.35 keV most of the luminosity is below this energy, especially if the shock is radiative, as is likely for shocks with velocities below ∼ 400 km s -1 (ref.

3). In addition, the line profiles of the optical emission show that there are shocks of lower velocities, most likely tangential shocks [START_REF] Pun | Modeling the Hubble Space Telescope Ultraviolet and Optical Spectrum of Spot 1 on the Circumstellar Ring of SN 1987A[END_REF] , which radiate mainly below the observable X-ray band and add to the ionizing flux. The typical shock velocity from the optical lines is ∼ 200 km s -1 (ref. 5), which corresponds to a temperature of ∼ 50 eV. Because the opacity increases as E -3 (Where E is the photon energy), the absorption efficiency of the ejecta of this low energy radiation is nearly 100%. The absorbed X-rays are therefore more than enough to explain the observed increase in the optical bands. A more accurate estimate of the spectral distribution would require a detailed numerical calculation of the X-ray absorption and thermalization, which is beyond the scope of this paper. We also remark that both the optical emission from the ring 5 and the mid-IR 8 are strongly correlated with the X-rays.

The conversion from X-rays to optical emission takes place first by photoelectric absorption, creating fast non-thermal electrons with energies up to keV. These are thermalized by excitation, ionization and heating of the free, thermal electrons. For hydrogen at the relevant degree of ionization, 35 -40% of the secondary electron energy goes into ionization, followed by recombination, while ∼ 8% goes into direct excitation of n > 2 (refs 39, 40). At 100 -200 K, ∼ 9% of the ionization energy and ∼ 16% of the excitation energy emerges as Hα (ref. 41). In total this corresponds to ∼ 5% of the X-ray energy. Other elements add to this. The relative importance of excitation, ionization and heating will remain roughly constant as long as the ionization is ∼ < 10 -2 , and we therefore do not expect conversion efficiency for the two bands to change over time. However, once the ionization surpasses this value, the fraction going into heating increases at the expense of the other channels. This will increase the far-IR fine structure lines compared to the optical lines until the temperature is high enough for thermal effects to increase the optical/UV emission [START_REF] Kozma | Late Spectral Evolution of SN 1987A. I. Temperature and Ionization[END_REF][START_REF] Kozma | Late Spectral Evolution of SN[END_REF] .

Effects of dust and positron leakage in the ejecta

There is strong evidence for dust in the ejecta from early observations [START_REF] Lucy | Dust Condensation in the Ejecta of Supernova 1987A -Part Two[END_REF][START_REF] Wooden | Airborne spectrophotometry of SN 1987A from 1.7 to 12.6 microns -Time history of the dust continuum and line emission[END_REF][START_REF] Moseley | Far-infrared observations of thermal dust emission from supernova 1987[END_REF] , as well as from observations discussed in this supplement. In principle, a transition from optically thick to optically thin dust could account for an increase in the flux. This is, however, unlikely for several reasons.

First, it would be a strange coincidence that this would occur at the same time as the X-ray flux becomes important. Secondly, compared to the flux from our model with constant covering fraction (i.e. optically thick dust clumps), which is given by the dashed line in Fig. 3, the increase has to be a factor of ∼ 8 in the R-band and ∼ 5 in the B-band. The estimated covering factor of the dust at early times was ∼ 0.5 (refs 23, 24), implying a maximum increase in the flux by a factor of ∼ 2.

Moreover, we expect the increase to occur first in the R-band and later in the B-band, based on the higher extinction in the latter band. The imaging also indicates dust obscuration in the central part of the SN up to the latest epochs (Supplementary Fig. 1), although this may also be an artefact of the external X-ray illumination, as discussed in the main paper.

Finally, positron leakage from the Fe-rich material containing the 44 Ti may enhance the emission from the O-and H-rich zones in the core [START_REF] Jerkstrand | The 44 Ti-powered spectrum of SN 1987A. Astron[END_REF] . This depends sensitively on the strength of a magnetic field in the ejecta, which is uncertain. The main effect of positron leakage is that the hydrogen and intermediate element lines would increase at the expense of the Fe I-II lines. At most this may enhance the emission in the observed bands by a factor of ∼ 2 compared to the complete trapping case. As discussed above, this is far from sufficient to explain the observed increase in the R-and B-bands. has been applied to the ACS and WFC3 fluxes in order to account for the differences between these instruments and WFPC2. The red and blue data points correspond to the same expanding elliptical aperture as in Fig. 2, shown here for reference. Green data points are for a fixed aperture that encloses as much of the ejecta as possible (sma = 0.50 ′′ , see also Fig. 1) while black data points are for an expanding circular aperture with radius equal to the semi-major axis of the expanding elliptical aperture. Open symbols indicate approximate flux levels after accounting for emission from the circumstellar ring. The light curve from the fixed aperture clearly levels off at late times. This is at least partly due to ejecta material expanding out of the aperture, but other effects may also contribute. Supplementary Figure 6: Ejecta velocity corresponding to optical depth τ = 1. The optical depth is measured from the ring as a function of energy at 10, 20 and 30 years after the explosion. The maximum velocity for each of these curves is given by the ejecta velocity in the plane of the ring, which is 2.1 × 10 4 (t/10 yrs) -1 km s -1 . Note the relatively small change in velocity with time of τ (E) = 1 for energies ∼ > 0.2 keV. The FWHM of the line profiles are therefore expected to decrease relatively slowly with time, as long as most of the shock energy is emitted above this energy. Note also that at 30 years the full ejecta will be transparent at ∼ > 6 keV.

  Figures

Figure 1 :

 1 Figure 1: A selection of HST R-band images. The observing dates are 1994-09-24 (a), 2000-11-

1 )Figure 2 :

 12 Figure 2: Light curves of the ejecta in different wavebands. Data points from WFPC2, ACS and

Figure 3 :

 3 Figure 3: Evolution of the luminosity from the ejecta in the R-and B-bands. The dashed black

Supplementary Figure 2 :

 2 Comparison of ejecta light curves for different apertures. Panel a shows the R-band and panel b shows the B-band (note the different scales on the y-axes). Data points from WFPC2, ACS and WFC3 are shown as dots, triangles and stars, respectively. A correction factor

Acknowledgments

This work was supported by the Swedish Research Council and the Swedish National Space Board. Support for the HST observing program was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc.

Supplementary Information

Supplementary information accompanies this paper.

Author contribution

J.L. carried out the data reduction and analysis together with G. Ö., P.G., B. L., J.S. and P.C.; C.F. performed the theoretical modelling together with A.J. and C.K.; J.L. and C.F. wrote the paper. R.P.K. is PI for the HST/SAINTS collaboration. All authors discussed the results and commented on the manuscript.

Author information

Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests. Readers are welcome to comment on the online version of this article at www.nature.com/nature. Correspondence and requests for materials should be directed to J.L. (josefin.larsson@astro.su.se) or C.F. (claes@astro.su.se).

Supplementary information 1 Photometry

Our analysis includes all HST imaging observations performed between 1994 and 2009 in the the red (WFPC2/F675W, ACS/F625W and WFC3/F675W) and blue (WFPC2/F439W, ACS/F435W and WFC3/F438W) broad-band filters (the two groups of filters will be referred to as the R-and B-bands from here on). The images were drizzled to combine dithered exposures and to remove cosmic rays. Details of the observations are given in Supplementary Table 1 and a selection of images are shown in Supplementary Fig. 1. The images show the expansion of the asymmetric ejecta and the brightening hot spots in the circumstellar ring.

We measured the brightness of the ejecta as a function of time using the IRAF/STSDAS task ELLIPSE, using different elliptical and circular apertures described below. The measurements give mean flux densities, which we convert into total fluxes by multiplying with the rectangular bandwidth of the different filters. When converting to total fluxes we also correct for the fact that there are some differences between the filters used for the three instruments. To determine the corrections we used the HST FOS spectrum from 1997 [START_REF] Chugai | Hubble Space Telescope Spectrum of SN 1987A at an Age of 8 Years: Radioactive Luminescence of Cool Gas[END_REF] (this covers the entire spectral range probed by all the filters) and used the SYNPHOT package to calculate flux densities in the different filters. After converting to total fluxes we obtain the following flux ratios: (ACS/F625W) / (WFPC2/F675W) = 1.08, (WFC3/F625W) / (WFPC2/F675W) = 1.16, 1 (ACS/F435W) / (WFPC2/F439W) = 1.27 and (WFC3/F438W) / (WFPC2/F439W) = 1.06. In all the light curves that we show we have corrected the ACS and WFC3 fluxes in accordance with this. A small offset still remains (see e.g. Fig. 2), which may be due to charge transfer efficiency losses in WFPC2 (see discussion below). Changes in the spectral energy distribution (SED) would also have some effect on the conversion factors, although we note that observations [START_REF] Chugai | Hubble Space Telescope Spectrum of SN 1987A at an Age of 8 Years: Radioactive Luminescence of Cool Gas[END_REF][START_REF] Wang | Hubble Space Telescope Spectroscopic Observations of the Ejecta of SN 1987A at 2000 Days[END_REF] suggest that the SED does not vary significantly with time.

For each of the observations we also determined the sky brightness using the IRAF/FITSKY routine. Specifically, we determined the centroid sky value in five empty regions located around SN 1987A and then used the average of these measurements to subtract the sky background from the light curve. The background is typically only a few per cent of the intrinsic ejecta brightness.

We also considered the possibility of contamination from the two bright stars that are located to the north-east and south-west of SN 1987A. These stars have strong diffraction spikes from the secondary-mirror spiders, which may pass through the ejecta depending on the HST roll angle. We inspected all the images and found that there is only one observation (2006-04-15) where there is a strong spike passing through our apertures. We estimated the contribution from this spike by measuring the brightness of the other spikes from the same star, and corrected the light curves accordingly. Another source of contamination is one of the narrow outer rings of SN 1987A, which is partly projected on top of the ejecta. However, the small, decreasing flux [START_REF] Tziamtzis | The outer rings of SN[END_REF] of this component is negligible compared to the emission from the ejecta.

The shape of the ejecta is roughly elliptical so as a first step we used an elliptical aperture leads to an underestimate of the flux in the B-band [START_REF] Jerkstrand | The 44 Ti-powered spectrum of SN 1987A. Astron[END_REF] .

An important test is that this model accurately reproduces the Hα evolution up to day 3,600 (ref.

2). A comparison with the spectrum at 7.8 years [START_REF] Chugai | Hubble Space Telescope Spectrum of SN 1987A at an Age of 8 Years: Radioactive Luminescence of Cool Gas[END_REF] shows agreement to within 50% for the R-band, but the model underestimates the flux in the B-band by a factor of 2.5. To model the shape of the light curves we therefore renormalize the B-band fluxes by a factor 2.5 and the R-band by a factor 1.5. The total energy input from radioactivity can be estimated to ∼ 1.2 × 10 36 erg s -1 at 20 years [START_REF] Jerkstrand | The 44 Ti-powered spectrum of SN 1987A. Astron[END_REF] . In addition to this there is a substantial contribution from delayed recombination (freeze-out). This comes primarily from the low-density hydrogen envelope and is seen mainly as Balmer emission. The decrease of this component is substantially faster than the 44 Ti decay and is responsible for the fast decline seen before ∼ 4, 000 days (Fig. 3).

When translating the broad-band fluxes to luminosities we assume a distance of 50 kpc and a reddening of E B-V = 0.16 mag [START_REF] Fitzpatrick | Observations of Sk-69 deg 203 and the interstellar extinction towards SN[END_REF] . For the comparison with the observations we use the light curve from the expanding elliptical aperture, multiplied by a constant factor for the R-and B-bands. The reason for using this, rather than the fixed aperture in Fig. 1, is that the latter shows a misleading flattening at epochs later than ∼ 7, 000 days because of the expansion of part of the ejecta outside the aperture. The constant factors were determined as an average of the ratio of the flux in the fixed aperture to that of the expanding aperture for epochs earlier than 7, 000 days. The constants are 2.6 for the R-band and 2.5 for the B-band. There is of course some uncertainty in the correction from the expanding aperture to the total emitted flux, which also affects the 44 Ti mass.

To determine the X-ray input to the ejecta we calculated the photoelectric absorption along

Supplementary Figures

Supplementary Figure 1


Fabian Walter
email: walter@mpia.de

Roberto Decarli

Chris Carilli

Frank Bertoldi

Pierre Cox

Elisabete Da Cunha

Emanuele Daddi

Mark Dickinson

Dennis Downes

David Elbaz

Richard Ellis

Jacqueline Hodge

Roberto Neri

Dominik Riechers

Axel Weiss

Eric Bell

Helmut Dannerbauer

Melanie Krips

Mark Krumholz

Lindley Lentati

Roberto Maiolino

Karl Menten

Hans-Walter Rix

Brant Robertson

Hyron Spinrad

Dan Stark

Daniel Stern

The Intense Starburst HDF850.1 in a Galaxy Overdensity at z=5.2 in the Hubble Deep Field

The Hubble Deep Field (HDF) is a region in the sky that provides one of the deepest multi-wavelength views of the distant universe and has led to the detection of thousands of galaxies seen throughout cosmic time [START_REF] Williams | The Hubble Deep Field: Observations, Data Reduction, and Galaxy Photometry[END_REF] . An early map of the HDF at a wavelength of 850 microns that is sensitive to dust emission powered by star formation revealed the brightest source in the field, dubbed HDF850.1 [START_REF] Hughes | A Submillimetre Survey of the Hubble Deep Field: Unveiling Dust-Enshrouded Star Formation in the Early Universe[END_REF] . For more than a decade, this source remained elusive and, despite significant efforts, no counterpart at shorter wavelengths, and thus no redshift, size or mass, could be identified [START_REF] Downes | Proposed identification of Hubble Deep Field submillimeter source HDF 850.1[END_REF][START_REF] Dunlop | Discovery of the galaxy counterpart of HDF 850.1, the brightest submillimetre source in the Hubble Deep Field[END_REF][START_REF] Wagg | A broad-band spectroscopic search for CO line emission in HDF850.1: the brightest submillimetre object in the Hubble Deep Field-North[END_REF][START_REF] Cowie | An Accurate Position for HDF 850.1: The Brightest Submillimeter Source in the Hubble Deep Field-North, Astroph[END_REF][START_REF] Carilli | The Radio-to-Submillimeter Spectral Index as a Redshift Indicator[END_REF] . Here we report, using a millimeter wave molecular line scan, an unambiguous redshift determination for HDF850.1 of z=5.183. This places HDF850.1 in a galaxy overdensity at z~5.2 in the HDF, corresponding to a cosmic age of only 1.1 Gyr after the Big Bang. This redshift is significantly higher than earlier estimates 3,4,6,8 and higher than most of the >100 sub-millimeter bright galaxies identified to date. The source has a star formation rate of 850 M sun yr -1 and is spatially resolved on scales of 5 kpc, with an implied dynamical mass of ~1.3x10 [START_REF] Alexander | The Chandra Deep Field North Survey. XIII. 2 Ms Point-Source Catalogs[END_REF] M sun , a significant fraction of which is present in the form of molecular gas. Despite our accurate redshift and position, a counterpart arising from starlight remains elusive.

We have obtained a full frequency scan of the 3 mm band towards the HDF using the IRAM Plateau de Bure Interferometer (PdBI). The observations covered the frequency range from 80-115 GHz in 10 frequency settings at uniform sensitivity and at a resolution (~2.3") that is a good match to galaxy sizes at high redshift. They resulted in the detection of two lines of Carbon Monoxide (CO), the most common tracer for molecular gas at high redshift [START_REF] Solomon | Molecular Gas at High Redshift[END_REF] , at 93.20 GHz and 111.84 GHz at the position of HDF850.1. Identifying these lines with the J=5 and J=6 rotational transitions of CO gives a redshift for HDF850.1 of z=5.183. This redshift was then unambiguously confirmed by the PdBI detection of the 158 μm line of ionized carbon ([CII], redshifted to 307.38 GHz), one of the main cooling lines of the star-forming interstellar medium. Stacking of other molecules covered by our frequency scan that trace higher volume densities did not lead to a detection (see Supplementary Information). Subsequently, the J=2 line of CO has also been detected using the NRAO Jansky Very Large Array (Jansky VLA) at 37.29 GHz. The observed [CII] and CO spectra towards HDF850.1 are shown in Fig. 1.

The beamsize of our CO observations (~2.3", 15 kpc at z=5.183) is too large to spatially resolve the molecular gas emission in HDF850.1. However the [CII] and underlying continuum observations (~1.2" x 0.8") show that the source is extended (hitherto, the interstellar medium has been spatially resolved only in extremely rare quasar host galaxies at such high redshift [START_REF] Walter | A kiloparsec-scale hyper-starburst in a quasar host less than 1 Gigayear after the Big Bang[END_REF] ). A single Gaussian fit yields a deconvolved size of 0.9±0.3", or 5.7±1.9 kpc at the redshift of the source. Fig. 2 shows the maps of total [CII] emission (left) as well as the red-and blue-shifted parts of the [CII] line (right) superposed on the deepest available Hubble Space Telescope (HST) images of the HDF [START_REF] Williams | The Hubble Deep Field: Observations, Data Reduction, and Galaxy Photometry[END_REF] . The derived dynamical mass is M dyn ~1.3±0.4 x 10 11 M sun assuming an arbitrary inclination of 30 degrees. An alternative interpretation is that the source is a merger of two galaxies, rather than a single rotating disk, which would lower the implied dynamical mass. Fig. 2 shows that the source is completely obscured in the observed optical and near-infrared wavebands (i.e. the rest-frame UV). There is no indication of HDF850.1 harboring an active galactic nucleus powered by a supermassive black hole (quasar) [START_REF] Alexander | The Chandra Deep Field North Survey. XIII. 2 Ms Point-Source Catalogs[END_REF] .

The CO(6-5)/CO(2-1) line luminosity ratio (in units of K km s -1 pc 2 ) 9 is 0.23±0.05. Assuming that the gas is being emitted from the same volume, this implies that the high-J CO emission is sub-thermally excited on galactic scales, less than seen in the nuclei of local starburst galaxies [START_REF] Loenen | Excitation of the molecular gas in the nuclear region of M 82[END_REF] . Using a standard large velocity gradient (LVG) model we find that the observed CO line intensities can be fit with a moderate molecular hydrogen density of 10 3.2 cm -3 and a kinetic temperature of 45 K for virialized clouds (dv/dr=1.2 km s -1 pc -1 ). We caution though that these numbers would change if the CO transitions were not emitted from the same volume. The predicted CO(1-0) line luminosity is 4.3 x 10 10 K km s -1 pc 2 , close to the measured value for CO (2-1). Depending on the choice of α, the CO-to-H 2 conversion factor, this line luminosity implies a molecular gas mass of M H2 = 3.5 x (α/0.8) x 10 10 M sun ; here α=0.8, in units of M sun (K km s -1 pc 2 ) -1 , is the conversion factor adopted for ultra-luminous infrared galaxies (ULIRGs) [START_REF] Downes | Rotating Nuclear Rings and Extreme Starbursts in Ultraluminous Galaxies[END_REF] and thought to be applicable to sub-millimeter bright objects [START_REF] Tacconi | Submillimeter Galaxies at z ~ 2: Evidence for Major Mergers and Constraints on Lifetimes, IMF, and CO-H2 Conversion Factor[END_REF] . The implied molecular gas mass fraction is M H2 /M dyn ~ 0.25±0.08 (α/0.8); i.e. even with a low ULIRG conversion factor the molecular gas constitutes a significant fraction of the overall dynamical mass. This molecular gas mass (and fraction) is comparable to what is found in other sub-millimeter bright galaxies that are typically located at much lower redshift [START_REF] Tacconi | Submillimeter Galaxies at z ~ 2: Evidence for Major Mergers and Constraints on Lifetimes, IMF, and CO-H2 Conversion Factor[END_REF][START_REF] Ivison | Tracing the molecular gas in distant submillimetre galaxies via CO(1-0) imaging with the Expanded Very Large Array[END_REF] .

The line-free channels of the observations (Fig. 1) were used to constrain the underlying continuum emission. Our accurate position of the rest-frame 158 μm emission is indicated as a cross in Fig. 2 (right). We combine our continuum detections at 307 GHz and 112 GHz with published values and new Herschel Space Telescope observations to constrain the far-infrared (FIR) properties of the source (see Supplementary Information for details). Our best fit gives a FIR luminosity of L FIR =6.5±1 x 10 12 L sun , a dust temperature of 35±5 K (i.e., broadly consistent with the average kinetic temperature of the molecular gas), a dust mass of M dust = 2.75±0.5 x 10 8 M sun and a star formation rate of 850 M sun yr -1 (with an uncertainty of ~30%). Given the extent of the source this results in an galaxy-averaged star formation rate surface density of 850 M sun yr -1 / (π x (2.8 kpc) 2 ) ~ 35 M sun yr -1 kpc -2 (uncertainty ~50%), more than an order of magnitude less than found in nearby merging systems and a compact quasar host galaxy at z=6.42 that has been studied in similar detail [START_REF] Walter | A kiloparsec-scale hyper-starburst in a quasar host less than 1 Gigayear after the Big Bang[END_REF] . HDF850.1 falls on the universal local star formation law that relates the average surface density of the star formation rate to that of the molecular gas mass per local free-fall time [START_REF] Krumholz | A Universal, Local Star Formation Law in Galactic Clouds, nearby Galaxies, High-redshift Disks, and Starbursts[END_REF] . The estimated surface density would increase if future observations resolved the source structure.

The resulting [CII]/FIR luminosity ratio of L [CII] /L FIR = 1.7 ± 0.5 x 10 -3 in HDF850.1 is comparable to what is found in normal local star-forming galaxies 17 , but is an order of magnitude higher than what is found in a z=6.42 quasar 10 , the only other high-z system where the [CII] emission could be resolved to date. Recent studies indicate that this ratio is a function of environment, with a low value (L [CII] /L FIR ~ 1 x 10 -4 ) for luminous systems dominated by a central black hole (quasars) and a high ratio (up to L [CII] /L FIR ~ 1 x 10 -2 ) for low-metallicity environments. Our relatively high ratio in L [CII] /L FIR is consistent with HDF850.1 being a high redshift star-forming system in a non-quasar environment 17 .

An inspection of the distribution of galaxies towards HDF850.1 that have spectroscopic redshifts shows that there is an overdensity of galaxies at the exact redshift of HDF850.1, including a quasar at z=5.186 [START_REF] Barger | X-ray, Optical, and Infrared Imaging and Spectral Properties of the 1Ms Chandra Deep Field North Sources[END_REF] (Fig. 3 and Supplementary Information). This makes this region one of the most distant galaxy overdensities known to date [START_REF] Capak | A massive protocluster of galaxies at a redshift of z~5[END_REF] . An elliptical galaxy at z=1.224 [START_REF] Barger | A Highly Complete Spectroscopic Survey of the GOODS-N Field1[END_REF] that is situated close to HDF850.1 in projection (~1" to the NE) could potentially act as a gravitational lens for this source [START_REF] Downes | Proposed identification of Hubble Deep Field submillimeter source HDF 850.1[END_REF][START_REF] Dunlop | Discovery of the galaxy counterpart of HDF 850.1, the brightest submillimetre source in the Hubble Deep Field[END_REF][START_REF] Hogg | A Candidate Gravitational Lens in the Hubble Deep Field[END_REF] . Using a velocity dispersion of 146 km s -1 in a singular isothermal sphere for this elliptical galaxy [START_REF] Dunlop | Discovery of the galaxy counterpart of HDF 850.1, the brightest submillimetre source in the Hubble Deep Field[END_REF] and our new redshift and position of HDF850.1, we derive an amplification factor of ~1.4. A similar flux amplification is found for a simple point source lens model with mass 3.5 x 10 11 M sun . This implies that even if lensing is occurring, the quantities derived here would not need to be revised significantly. HDF850.1 remains outstanding in the study of dust-obscured starbursts at high redshift, being one of the first such sources discovered, and yet evading detection in the optical and near-infrared. Its redshift of z=5.183 enforces the presence of a high redshift tail (z>4) of sub-millimeter bright star-forming (non-AGN/quasar) galaxies (currently there are only about half a dozen systems known) [START_REF] Riechers | A Massive Molecular Gas Reservoir in the z = 5.3 Submillimeter Galaxy AzTEC-3[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] Schinnerer | Molecular Gas in a Submillimeter Galaxy at z = 4.5: Evidence for a Major Merger at 1 Billion Years after the Big Bang[END_REF][START_REF] Combes | A bright z = 5.2 lensed submillimeter galaxy in the field of Abell 773. HLSJ091828.6+514223[END_REF][26] . Only a small fraction of sub-millimeter bright sources is expected to be at very high redshift [START_REF] Ivison | A robust sample of submillimetre galaxies: constraints on the prevalence of dusty, high-redshift starbursts[END_REF] ---it is thus ironic that the first blank-field source belongs to this subgroup. HDF850.1's large spatial extent, in combination with the modest CO excitation, a moderate surface density of its star formation rate, and a high [CII]/FIR luminosity ratio all point to the presence of a spatially extended major starburst that is completely obscured even in the deepest HST images available for the HDF. The absence of a possible counterpart in the available deep imaging, even though the star-forming interstellar medium is distributed over many square kpc, makes this source extreme [START_REF] Riechers | A Massive Molecular Gas Reservoir in the z = 5.3 Submillimeter Galaxy AzTEC-3[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] Schinnerer | Molecular Gas in a Submillimeter Galaxy at z = 4.5: Evidence for a Major Merger at 1 Billion Years after the Big Bang[END_REF] . Given its high molecular gas mass (3.5 x (α/0.8) x 10 10 M sun ) and star formation rate (850 M sun yr -1 ) HDF850.1 can build a significant stellar component as early as z~4 [START_REF] Wiklind | A Population of Massive and Evolved Galaxies at[END_REF] (~few hundred Myr from z~5). Blind line searches through spectral scans at millimeter wavelengths as performed here thus play a fundamental role in unveiling the nature of star-forming galaxies that are completely obscured in the (restframe) optical and UV even if multi-wavelength data at unparalleled depth are available.

Figures

Figure 1: Detection of four lines tracing the star-forming interstellar medium in HDF850.1. From top to bottom: [CII], ν obs = 307.383 GHz; CO(6-5), ν obs = 111.835 GHz; CO(5-4), ν obs = 93.202 GHz, CO(2-1), ν obs = 37.286 GHz. Zero velocity corresponds to a redshift of z=5.183. Continuum emission is detected in the top two panels at 6.80±0.8 mJy and 0.13±0.03 mJy, respectively. We derive a 3σ continuum limit of 30 µJy from the Jansky VLA observations at 37.3 GHz using a bandwidth larger than shown here. Gaussian fits to the lines give a full width at half maximum (FWHM) of 400±30 km s -1 , narrower than typically found in sub-millimeter selected galaxies [START_REF] Downes | Rotating Nuclear Rings and Extreme Starbursts in Ultraluminous Galaxies[END_REF] . The observed integrated line flux densities are: S[CII]=14.6±0.3 Jy km s -1 , S[CO(6-5)]=0.39±0.1 Jy km s -1 , S[CO(5-4)]=0.50±0.1 Jy km s -1 and S[CO(2-1)]=0.17±0.04 Jy km s -1 . The resulting line luminosities are 9 : 5.0, 1.0, 1.9 and 4.1 x 10 10 K km s -1 pc 2 or 1,104, 10.6, 11.4 and 1.5 x 10 7 L sun (uncertainties as given for integrated line flux densities). LVG modeling gives a predicted CO(1-0) line luminosity of 4.3 x 10 10 K km s -1 pc 2 . [START_REF] Dickinson | The Unusual Infrared Object HDF-N J123656.3+621322[END_REF] . The cross indicates the position and its 5σ uncertainty of the restframe 158 μm continuum emission peak (RA: 12h36m51.976s, DEC: 62 o 12'25.80'' in the J2000.0 system), consistent with earlier millimeter interferometric measurements [START_REF] Downes | Proposed identification of Hubble Deep Field submillimeter source HDF 850.1[END_REF][START_REF] Cowie | An Accurate Position for HDF 850.1: The Brightest Submillimeter Source in the Hubble Deep Field-North, Astroph[END_REF] at lower resolution. The [CII] contours have been derived by averaging the spectrum (Fig. 1) from -400 km s -1 to 0 km s -1 and 0 km s -1 to +400 km s -1 and are plotted at levels of 7, 10 and 13 mJy beam -1 (1σ=1.8 mJy beam -1 ), respectively. In both panels the beamsize of the [CII] observations (1.23" x 0.81") is indicated in the bottom left corner. From the spatial offset (total offset=0.9", i.e. radius: 0.45", or r=2.8 kpc) and the FWHM of the line, we derive an approximate dynamical mass of M dyn ~ 3.4 x 10 10 M sun / (sin i) [START_REF] Hughes | A Submillimetre Survey of the Hubble Deep Field: Unveiling Dust-Enshrouded Star Formation in the Early Universe[END_REF] where i is the (unknown) inclination of the system (using M dyn sin 2 i = 1.3 x (FWHM/2) 2 r/G, where G is the gravitational constant [START_REF] Daddi | Very High Gas Fractions and Extended Gas Reservoirs in z = 1.5 Disk Galaxies[END_REF] ). These deep HST images of the HDF fail to reveal the (rest-frame) UV/optical counterpart of the galaxy that is forming stars at a rate of ~850 M sun yr -1 .

[CII] contours on I-band HDF850.1 red/blue-shifted [CII] on J-band

Figure 3: Distribution of galaxies near HDF850.1 Left: Distribution of spectroscopic redshifts towards the HDF and its surrounding (GOODS-N). HDF850.1 is indicated by a red color, the quasar at the same redshift [START_REF] Barger | X-ray, Optical, and Infrared Imaging and Spectral Properties of the 1Ms Chandra Deep Field North Sources[END_REF] is shown in blue. There is an overdensity of galaxies in the redshift bin that contains HDF850.1. The high source density at z~5.7 is an observational artefact due to narrow-band Lyman-α imaging surveys of the region (with spectroscopic follow-up) that are sensitive to this particular narrow redshift range.

Right: Spatial coverage of the sources in the redshift bin z=5.183-5.213. The small border indicates the size of the HDF -the larger border shows the surrounding area of GOODS-N. The presence of a strongly star-forming galaxy (HDF850.1) and a quasar [START_REF] Barger | X-ray, Optical, and Infrared Imaging and Spectral Properties of the 1Ms Chandra Deep Field North Sources[END_REF] in this region provides evidence for cosmic structure formation in the first Gyr of the universe. See Supplemental Information for more details.

The redshifts of the galaxies shown in the left panel of Figure 3 have been compiled from a number of published spectroscopic surveys of the GOODS-N region [START_REF] Barger | A Highly Complete Spectroscopic Survey of the GOODS-N Field1[END_REF][START_REF] Cohen | Redshift Clustering in the Hubble Deep Field[END_REF][START_REF] Cohen | Caltech Faint Galaxy Redshift Survey. X. A Redshift Survey in the Region of the Hubble Deep Field North[END_REF][START_REF] Cohen | Caltech Faint Galaxy Redshift Survey. XIII. Spectral Energy Distributions for Galaxies in the Region of the Hubble Deep Field North[END_REF][START_REF] Dawson | Serendipitously Detected Galaxies in the Hubble Deep Field[END_REF][START_REF] Kakazu | Mapping Extremely Low Metallicity Galaxies to Redshift One[END_REF][START_REF] Hu | An Atlas of z=5.7 and z=6.5 Lyman Alpha Emitters[END_REF][START_REF] Stark | Keck spectroscopy of faint 3<z<7 Lyman break galaxies -I. New constraints on cosmic reionization from the luminosity and redshift-dependent fraction of Lyman Alpha emission[END_REF][START_REF] Stark | Keck Spectroscopy of Faint 3<z<7 Lyman Break Galaxies: A High Fraction of Line Emitters at Redshift Six[END_REF] and include new data from ongoing surveys (Stark et al. in prep., Stern et al. in prep). The redshifts shown in the histogram have primarily been derived from the red-shifted Lyman-Alpha emission line (typically, asymmetric line profiles are seen in the spectra, which add confidence to their high redshift). The histogram shown in Figure 3 (left) only includes the highest quality targets from these surveys.

Table 1 shows the coordinates and redshifts for the overdensity centered on the histogram bin that contains HDF850.1. The redshifts are typically accurate to the third digit. 12 sources (13 including HDF850.1) are distributed over a narrow redshift range between 5.183<z<5.213 (Δz = 0.03). This corresponds to a difference in comoving radial distance of 15.7 Mpc (i.e. a proper distance of 2.5 Mpc). If one assumes spherical symmetry for the overdensity this distance would correspond to a projected extent of 6.6 arcmin, which is close to what is observed (Fig. 3, right).

We note that HDF 850.1 and the quasar are amongst the lowest redshift galaxies in the overdensity. One possible explanation is that Lyman Alpha emission lines (used to derive the redshifts of the remaining galaxies shown here) tend to be redshifted from the systemic velocity due to systematic absorption on the blue side of the line at these redshifts (the redshift determination of HDF850.1 through molecular emission lines is not affected by this bias).

Spectral Energy Distribution of HDF 850.1

The observations presented here provide new constraints on the spectral energy distribution (SED) of HDF850.1. Figure S1 below summarizes all measurements (including data from the literature [START_REF] Hughes | A Submillimetre Survey of the Hubble Deep Field: Unveiling Dust-Enshrouded Star Formation in the Early Universe[END_REF][START_REF] Downes | Proposed identification of Hubble Deep Field submillimeter source HDF 850.1[END_REF][START_REF] Cowie | An Accurate Position for HDF 850.1: The Brightest Submillimeter Source in the Hubble Deep Field-North, Astroph[END_REF][START_REF] Pope | The Hubble Deep Field-North SCUBA Super-map -IV. Characterizing submillimetre galaxies using deep Spitzer imaging[END_REF] ) and includes new deep Herschel data of the region at 100, 160, 250, 350 and 500 microns [START_REF] Elbaz | GOODS-Herschel: an infrared main sequence for star-forming galaxies[END_REF] . For the Herschel bands we plot the measured flux density at the position of the source (0.18, 0.22, 5.0, 4.8 and 6.0 mJy, respectively) and add to this 2 times the noise (σ=0.5, 1.1, 3.2, 4.0, 4.0 mJy in the region around HDF850.1, respectively) as upper limits. We note that even though HDF850.1 is forming stars at a high rate, it is barely detected in the deep Herschel observations. The measurements done with the Spitzer Space telescope (shown in green) are blended with the bright foreground elliptical and thus only upper limits can be derived for HDF850.1 [START_REF] Pope | The Hubble Deep Field-North SCUBA Super-map -IV. Characterizing submillimetre galaxies using deep Spitzer imaging[END_REF] . The full line shows our best-fit SED using a simple model that provides a consistent energy balance between the attenuated stellar emission and the dust emission [START_REF] Da Cunha | A simple model to interpret the ultraviolet, optical and infrared emission from galaxies[END_REF][START_REF] Da Cunha | Exploring the physical properties of local star-forming ULIRGs from the ultraviolet to the infrared[END_REF] . Our modeling shows that a minimum dust attenuation (defined as the difference between the observed and intrinsic magnitude at a given wavelength,

A λ = m λ[ obs ] -m λ[ model ]
) of A 1500Å ~ 5 mag in the far-UV and A 5500Å ~ 2 mag in the Vband is required to be consistent with the Spitzer/IRAC upper limits, which sample the rest-frame optical. The only other high-redshift source with similar properties (no detection in rest-frame UV/optical) is GN10 [START_REF] Daddi | CO Emission Line from the Optical and Near-IR Undetected Submillimeter Galaxy GN10[END_REF] . We note that an Arp220-like SED (shown as a dashed line) would result in even higher extinction values. An M82-like template (not shown), on the other hand, would be too bright in the rest-frame optical with respect to the observed upper limits. The emission by dust is described by using multiple components, including polycyclic aromatic hydrocarbon (PAH) features and a hot dust component in the mid-infrared, and dust in thermal equilibrium in the far-infrared, described by modified black bodies with an emissivity index of β=2.5. Lower values of β are difficult to reconcile with the observed steep drop in the Raleigh-Jeans part of the spectrum. This results in a total infrared luminosity of L IR = 8.7±1 x 10 12 L sun (FIR[42-122 µm] luminosity: L FIR = 6.5±1 x 10 12 L sun ) and an average dust temperature of T=35±5 K. The implied total dust mass is M dust = 2.75±0.5 x 10 8 M sun , with a resulting molecular gas-to-dust ratio of 130±30(α/0.8), similar to what is found in local galaxies [START_REF] Draine | Dust Masses, PAH Abundances, and Starlight Intensities in the SINGS Galaxy Sample[END_REF] within the uncertainties. The implied star formation rate is 850±100 M sun yr -1 (assuming a Chabrier stellar initial mass function). We note that the absence of reliable measurements in the rest-frame infrared around the peak of the dust emission make it difficult to obtain an accurate redshift based on photometry alone, in particular if the FIR measurements have significant uncertainties [START_REF] Wagg | A broad-band spectroscopic search for CO line emission in HDF850.1: the brightest submillimetre object in the Hubble Deep Field-North[END_REF] .

Figure S1:

Spectral energy distribution (SED) of HDF850.1, including all available measurements from the literature and this work. The full line is our best-fit model (see text for details) -the dashed line is the scaled SED of Arp220 that is typically used to fit SEDs of high-redshift starburst galaxies. See Supplementary Information for details.

Stacking of fainter lines in the frequency scan

Our full frequency scan covers rest frequencies of 490-710 GHz for the redshift of HDF850.1. CO is by far the brightest emission line in this range, and we do not expect to detect the next brightest set of high-density tracers individually. We have however attempted to obtain a statistical detection of these lines through stacking. Figure S2 shows the result of the stacking exercise for the high-density tracers HCN(6-5), HCN (7-6), HCN(8-7), HNC(6-5), HNC (7-6), HCO+(6-5), HCO+ (7-6), CS (11-10), CS (12-11), CS (13-12), CS(14-13), HOC + (6-5), HOC+(7-6) that are covered in our frequency scan.

No emission is detected in this stack with a 1σ flux limit of 0.06 Jy km s -1 (assuming a linewidth of 400 km s -1 ). This non-detection is consistent with the few detections of dense molecular gas tracers in high-redshift objects that are significantly fainter than the CO emission [START_REF] Barvainis | Multiple CO Transitions, CI, and HCN from the Cloverleaf Quasar[END_REF][START_REF] Solomon | The essential signature of a massive starburst in a distant quasar[END_REF][START_REF] Wagg | 5-4 Emission in APM 08279+5255 at z = 3.91[END_REF][START_REF] Riechers | First Detection of HCO+ at High Redshift[END_REF][START_REF] Garcia-Burillo | A New Probe of Dense Gas at High Redshift: Detection of HCO+(5-4) Emission in APM 08279+5255[END_REF][START_REF] Riechers | Detection of Emission from the CN Radical in the Cloverleaf Quasar at z=2.56[END_REF][START_REF] Guelin | Detection of HNC and tentative detection of CN at z = 3.9[END_REF][START_REF] Riechers | Dense Molecular Gas Excitation in Nuclear Starbursts at High Redshift: HCN, HNC, and HCO+(J=6-5) Emission in the z = 3.91 Quasar Host of APM 08279+5255[END_REF][START_REF] Danielson | The properties of the interstellar medium within a starforming galaxy at z= 2.3[END_REF][START_REF] Riechers | Dense Molecular Gas Excitation at High Redshift: Detection of HCO + (J=4-3) Emission in the Cloverleaf Quasar[END_REF] . Also, no stacked detection was achieved using different combinations of high density tracers (e.g. stacking only on HCN, HNC and HCO + , or including additional species such as CN, CCH, and HC3N) or stacking on the CO isotopomers [START_REF] Downes | Rotating Nuclear Rings and Extreme Starbursts in Ultraluminous Galaxies[END_REF] CO and C 18 O. Additional References:

Figure 2 :

 2 Figure 2: [CII] line emission towards HDF850.1. Left: [CII] contours on top of a deep HST image 1 of the region in a filter (I band) that covers the Ly-α line and UV continuum at z=5.183. [CII] contours show the averaged emission over 700 km s -1 and are plotted at 5, 7, 9, 11 mJy beam -1 (1σ=1.3 mJy beam -1 ). A Gaussian fit to the emission gives a deconvolved size of 0.9±0.3" or 5.7±1.9 kpc at z=5.183. The underlying continuum emission (not shown) is also extended on the same scales. Right: The blue and red contours indicate the approaching and receding [CII] emission relative to the systemic redshift of z=5.183. The color shows a deep HST image in a longer wavelength filter (NICMOS J band)[START_REF] Dickinson | The Unusual Infrared Object HDF-N J123656.3+621322[END_REF] . The cross indicates the position and its 5σ uncertainty of the restframe 158 μm continuum emission peak (RA: 12h36m51.976s, DEC: 62 o 12'25.80'' in the J2000.0 system), consistent with earlier millimeter interferometric measurements[START_REF] Downes | Proposed identification of Hubble Deep Field submillimeter source HDF 850.1[END_REF][START_REF] Cowie | An Accurate Position for HDF 850.1: The Brightest Submillimeter Source in the Hubble Deep Field-North, Astroph[END_REF] at lower resolution. The [CII] contours have been derived by averaging the spectrum (Fig.1) from -400 km s -1 to 0 km s -1 and 0 km s -1 to +400 km s -1 and are plotted at levels of 7, 10 and 13 mJy beam -1 (1σ=1.8 mJy beam -1 ), respectively. In both panels the beamsize of the [CII] observations (1.23" x 0.81") is indicated in the bottom left corner. From the spatial offset (total offset=0.9", i.e. radius: 0.45", or r=2.8 kpc) and the FWHM of the line, we derive an approximate dynamical mass of M dyn ~ 3.4 x 10 10 M sun / (sin i)[START_REF] Hughes | A Submillimetre Survey of the Hubble Deep Field: Unveiling Dust-Enshrouded Star Formation in the Early Universe[END_REF] where i is the (unknown) inclination of the system (using M dyn sin 2 i = 1.3 x (FWHM/2) 2 r/G, where G is the gravitational constant[START_REF] Daddi | Very High Gas Fractions and Extended Gas Reservoirs in z = 1.5 Disk Galaxies[END_REF] ). These deep HST images of the HDF fail to reveal the (rest-frame) UV/optical counterpart of the galaxy that is forming stars at a rate of ~850 M sun yr -1 .

Figure S2 :

 S2 Figure S2: Spectral line stack for the high-density tracers (HCN, HNC, HCO + , CS, HOC + ) covered by our frequency scan. The grey lines show the individual spectra before stacking whereas the black line shows the stacked result. No emission is detection in these lines (nor in any other combination of lines, see text).

Table 1 :

 1 List of sources in galaxy overdensity around HDF850.1. The last column shows the redshift reference (Stern: Stern et al., in prep., Stark: Stark et al., in prep.)

	RA(2000.0)	DEC(2000.0)	redshift z	Reference
	12:36:00.0	62:12:26.1	5.199	Stern
	12:36:26.5	62:12:07.4	5.200	Stark
	12:36:37.5	62:12:36.0	5.185	Stark
	12:36:39.8	62:09:49.1	5.187	Stark
	12:36:48.0	62:09:41.4	5.186	18
	12:36:49.2	62:15:38.6	5.189	Stern, Stark
	12:36:52.0	62:12:25.8	5.183	this work
	12:36:55.4	62:15:48.8	5.190	Stark
	12:36:55.5	62:15:32.8	5.191	Stern, Stark
	12:37:03.3	62:13:31.5	5.213	Stern, Stark
	12:37:09.9	62:15:31.1	5.191	Stark
	12:37:11.1	62:16:38.6	5.208	Stern
	12:37:15.6	62:16:23.6	5.189	Stern

Acknowledgements: This work is based on observations carried out with the IRAM

is supported by MPG (Germany), INSU/CNRS (France) and IGN (Spain). The Jansky Very Large Array of the National Radio Astronomy Observatory (NRAO) is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. DR acknowledges support from NASA through a Spitzer Space Telescope grant. RD acknowledges funding through DLR project FKZ 50OR1004. Contributions: F.W. had the overall lead of the project. The PdBI data were analyzed by R.D., F.W., P.C., R.N., M.K. and D.D. The Jansky VLA data reduction was performed by C.C., J.

Supplementary Information

1. Details on the Galaxy Overdensity at z=5.2