A deep ALMA image of the Hubble Ultra Deep Field

We present the results of the first, deep Atacama Large Millimeter Array(ALMA) imaging covering the full ≃4.5 arcmin2 of theHubble Ultra Deep Field (HUDF) imaged with Wide Field Camera 3/IR onHST. Using a 45-pointing mosaic, we have obtained a homogeneous 1.3-mmimage reaching σ1.3 ≃ 35 μJy, at a resolutionof ≃0.7 arcsec. From an initial list of ≃50 > 3.5σpeaks, a rigorous analysis confirms 16 sources with S1.3 >120 μJy. All of these have secure galaxy counterparts with robustredshifts ( = 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 formationrates (SFR). Our results show that stellar mass is the best predictor ofSFR in the high-redshift Universe; indeed at z ≥ 2 our ALMA samplecontains seven of the nine galaxies in the HUDF with M* ≥2 × 1010 M⊙, and we detect only onegalaxy at z > 3.5, reflecting the rapid drop-off of high-massgalaxies with increasing redshift. The detections, coupled withstacking, allow us to probe the redshift/mass distribution of the 1.3-mmbackground down to S1.3 ≃ 10 μJy. We find strongevidence for a steep star-forming `main sequence' at z ≃ 2, withSFR ∝M* and a mean specific SFR ≃ 2.2Gyr-1. Moreover, we find that ≃85 per cent of total starformation at z ≃ 2 is enshrouded in dust, with ≃65 per cent ofall star formation at this epoch occurring in high-mass galaxies(M* > 2 × 1010 M⊙), forwhich the average obscured:unobscured SF ratio is ≃200. Finally, werevisit the cosmic evolution of SFR density; we find this peaks at z≃ 2.5, and that the star-forming Universe transits from primarilyunobscured to primarily obscured at z ≃ 4.