ALMACAL III: A combined ALMA and MUSE Survey for Neutral, Molecular, and Ionised Gas in an HI-Absorption-Selected System

Studying the flow of baryons into and out of galaxies is an important part of understanding the evolution of galaxies over time. We present a detailed case study of the environment around an intervening Ly $\alpha$ absorption line system at $z_{\mathrm{a}\mathrm{b}\mathrm{s}}=0.633$, seen towards the quasar J0423$-$0130 ($z_{\rm QSO} = 0.915$)\mathrm{.}WedetectwithALMAthe$^{12}$CO(2--1),$^{12}$CO(3--2)and$1.2$mmcontinuumemissionfromagalaxyattheredshiftoftheLy$\alpha$absorberataprojecteddistanceof$135$ kpc. From the ALMA detections, we infer ISM conditions similar to those in low redshift Luminous Infrared Galaxies. DDT MUSE integral field unit observations reveal the optical counterpart of the ${}^{12}$CO emission line source and three additional emission line galaxies at the absorber redshift, which together form a galaxy group. The ${}^{12}$CO emission line detections originate from the most massive galaxy in this group. While we cannot exclude that we miss a fainter host, we reach a dust-uncorrected star-formation rate (SFR) limit of > $0.3 $\text{M}$_{\odot} $\text{ yr}$^{-1}$within$100$ kpc from the sightline to the background quasar. We measure the dust-corrected SFR (ranging from $3$ to $50$ M${}_{\odot }$ yr${}^{-1}$), the morpho-kinematics and the metallicities of the four group galaxies to understand the relation between the group and the neutral gas probed in absorption. We find that the Ly $\alpha$ absorber traces either an outflow from the most massive galaxy or intra-group gas. This case study illustrates the power of combining ALMA and MUSE to obtain a census of the cool baryons in a bounded structure at intermediate redshift.