We present the results of the first, deep ALMA imaging covering the full 4.5 arcmin 2 of the HUDF imaged with WFC3/IR on HST. Using a 45-pointing mosaic, we have obtained a homogeneous 1.3-mm image reaching σ 1.335 µJy, at a resolution of 0.7 arcsec. From an initial list of 50 > 3.5σ peaks, a rigorous analysis confirms 16 sources with S 1.3 > 120 µJy. All of these have secure galaxy counterparts with robust redshifts ( z = 2.15). Due to the unparalleled supporting data, the physical properties of the ALMA sources are well constrained, including their stellar masses (M * ) and UV+FIR star-formation rates (SFR). Our results show that stellar mass is the best predictor of SFR in the high-redshift Universe; indeed at z ≥ 2 our ALMA sample contains 7 of the 9 galaxies in the HUDF with M * ≥ 2 × 10 10 M , and we detect only one galaxy at z > 3.5, reflecting the rapid drop-off of high-mass galaxies with increasing redshift. The detections, coupled with stacking, allow us to probe the redshift/mass distribution of the 1.3-mm background down to S 1.3 10 µJy. We find strong evidence for a steep star-forming 'main sequence' at z 2, with SFR ∝ M * and a mean specific SFR 2.2 Gyr −1 . Moreover, we find that 85% of total star formation at z 2 is enshrouded in dust, with 65% of all star formation at this epoch occurring in high-mass galaxies (M * > 2 × 10 10 M ), for which the average obscured:unobscured SF ratio is 200. Finally, we revisit the cosmic evolution of SFR density; we find this peaks at z 2.5, and that the star-forming Universe transits from primarily unobscured to primarily obscured at z 4.

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The evolving far-IR galaxy luminosity function and dust-obscured star formation rate density out to ≃', Monthly