A deep ALMA Band 3 survey of HDFS/MUSE3D

Survey description and initial results

¹ Joint ALMA Observatory Alonso de Córdova 3107 Vitacura 763-0355 Santiago, Chile
² European Southern Observatory, Alonso de Córdova 3107 Vitacura Casilla 19001 Santiago de Chile, Chile
³ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y 455 Ciencias, Universidad Diego Portales Av. Ejército Libertador 441 Santiago Chile
⁴ European Southern Observatory Karl-Schwarzschild-Str. 2 85748 Garching Germany
⁵ National Radio Astronomy Observatory 520 Edgemont Road Charlottesville VA, 22903 USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 31 October 2024
Accepted: 19 November 2025

Abstract

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.