2026A&A...706A.236M

Context. After more than 10 years of ALMA operations, the community interest in conducting deep extra-galactic millimetre surveys resulted in varying strategic compromises between areal size and map depth to survey the sky. The current bias leans towards a galaxy population found in the field or towards rich starbursty proto-cluster groups, which are both tendentiously surveyed at coarse spatial resolutions.
Aims. We describe a survey that addresses these biases. A deep 3 mm survey was conducted with ALMA at long baselines in a 1 × 1 arcmin² region in the Hubble Deep Field South (HDFS) that was also covered by the Multi Unit Spectroscopic Explorer (MUSE) in order to assess resolved molecular gas properties in galaxies in group environments at z > 1.
Methods. ALMA observations comprising a four-pointing mosaic with a single band 3 (3 mm) spectral tuning were conducted to cover CO transitions from different groups identified by MUSE. This work consists of a total effective on-source time of 61 hours in configurations with baselines up to 15 km.
Results. The final dataset yields an angular resolution of 0.15″-0.2″ (depending on the imaging weights) and maximum recoverable scales of 1″-2″. The final continuum map reaches an unprecedented sensitivity of RMS ∼ 2 μJy/beam, which allowed us to detect three sources at 3 mm (only one of which with multi-wavelength counterparts from the rest-frame UV to radio). Moreover, we detected six line emitters associated with CO J = 2 - 1 at z_spec = 1.284, one of which was not previously detected by MUSE, and none of which was detected in 3 mm continuum. The inter-stellar medium gas masses range from ∼2 × 10⁹ to ∼9 × 10¹⁰ M_⊙ (adopting α_CO = 4 M_⊙/(K.km/s.pc²), including helium). This galaxy group is quite diverse overall, and no two galaxies are alike. Some are clearly offset physically with respect to Hubble imaging that traces the rest-frame ultra-violet emission. We also derived cosmic molecular gas mass densities using this sample as a reference for group environments and found that their densities are similar to that of the galaxy population found in field environments.