An optimal ALMA image of the Hubble Ultra Deep Field in the era of JWST: obscured star formation and the cosmic far-infrared background

We combine archival ALMA data targeting the Hubble Ultra Deep Field (HUDF) to produce the deepest currently attainable 1-mm maps of this key field. Our deepest map covers 4.2 arcmin2 , with a beamsize of 1.49 arcsec × 1.07 arcsec at an effective frequency of 243 GHz (1.23 mm). It reaches an rms of 4.6 𝜇Jy beam−1 , with 1.5 arcmin2 below 9.0 𝜇Jy beam−1 , an improvement of > 5 per cent (and up to 50 per cent in some regions) over the best previous map. We also make a wider, shallower map, covering 25.4 arcmin2 . We detect 45 galaxies in the deep map down to 3.6𝜎, 10 more than previously detected, and 39 of these galaxies have a JWST counterpart. A stacking analysis on the positions of ALMA-undetected JWST galaxies with z < 4 and stellar masses from 108.4 to 1010.4 M⊙ yields 10 per cent more signal compared to previous stacking analyses, and we find that detected sources plus stacking contribute (10.0±0.5) Jy deg−2 to the cosmic infrared background (CIB) at 1.23 mm. Although this is short of the (uncertain) background level of about 20 Jy deg−2 , we show that our measurement is consistent with the background if the HUDF is a mild (∼ 2𝜎) negative CIB fluctuation. This suggests that within the HUDF, JWST may have detected essentially all of the galaxies that contribute to the CIB. Our stacking analysis predicts that the field contains about 60 additional 15 𝜇Jy galaxies, and over 300 galaxies at the few 𝜇Jy level. However, the contribution of these fainter objects to the background is small, and converging, as anticipated from the now well-established strong correlation between galaxy stellar mass and obscured star formation.