The MUSE Ultra Deep Field (MUDF): VII. Probing high-redshift gas structures in the surroundings of ALMA-identified massive dusty galaxies

We present new ALMA continuum and spectral observations of the MUSE Ultra Deep Field (MUDF), a 2 × 2 arcmin2 region with ultradeep multiwavelength imaging and spectroscopy hosting two bright z ≈ 3:22 quasars used to study intervening gas structures in absorption. Through a blind search for dusty galaxies, we identified a total of seven high-confidence sources, six of which have secure spectroscopic redshifts. We estimate galaxy dust and stellar masses (Mdust ≃ 107:8-8:6 M⊙, M*≃ 1010:2-10:7 M⊙), as well as star formation rates (SFR ≃ 101:2-2:0 M⊙ yr-1) which show that most of these galaxies are massive and dust-obscured resembling (sub)millimeter galaxies at similar epochs. All six spectroscopically confirmed galaxies are within 500 km s-1 of metal absorption lines observed in the quasar sightlines, corresponding to 100% association rate. We also find that four of these galaxies belong to groups in which they are among the most massive members. The galaxies identified with ALMA are rarely found close in projection to the background quasars, likely due to the modest surface density of this population. Consequently, most of the absorbers observed in the quasar spectra originate from gas distributed within large-scale structures or from the CGM of other group members surrounding these dusty star-forming systems. While ALMA-detected sources are not always the nearest in spatial projection, they frequently align closely in velocity space (≤50 km s-1) with the absorption centroids. This suggests that these massive galaxies reside at the center of the gravitational potential wells of the gas structures traced in absorption.