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

TL;DR

This study combines deep ALMA and JWST data to map the Hubble Ultra Deep Field at 1.23mm, detecting more galaxies and constraining their contribution to the cosmic infrared background, suggesting JWST captures most obscured star formation.

Contribution

It provides the deepest ALMA map of the HUDF, detects additional galaxies, and offers new insights into the contribution of faint objects to the cosmic infrared background.

Findings

Detected 45 galaxies in the deep map, 10 more than previous studies.

Contributed approximately 10% to the cosmic infrared background at 1.23mm.

Predicted about 60 more galaxies at 15uJy and over 300 at a few uJy level.

Abstract

We combine archival ALMA data targeting the Hubble Ultra Deep Field (HUDF) to produce the deepest currently attainable 1-mm maps of this key region. Our deepest map covers 4.2arcmin^2, with a beamsize of 1.49''x1.07'' at an effective frequency of 243GHz (1.23mm). It reaches an rms of 4.6uJy/beam, with 1.5arcmin^2 below 9.0uJy/beam, an improvement of >5% (and up to 50% in some regions) over the best previous map. We also make a wider, shallower map, covering 25.4arcmin^2. We detect 45 galaxies in the deep map down to 3.6sigma, 10 more than previously detected, and 39 of these galaxies have JWST counterparts. A stacking analysis on the positions of ALMA-undetected JWST galaxies with z<4 and stellar masses from 10^8.4 to 10^10.4 M_sun yields 10% 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 1.23mm. Although this is short of the (uncertain) background level of about 20Jy/deg^2, we show that our measurement is consistent with the background if the HUDF is a mild (~2sigma) negative CIB fluctuation, and that the contribution from faint undetected objects is small and converging. In particular, we predict that the field contains about 60 additional 15uJy galaxies, and over 300 galaxies at the few uJy level. This suggests that JWST has detected essentially all of the galaxies that contribute to the CIB, as anticipated from the strong correlation between galaxy stellar mass and obscured star formation.