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

TL;DR

This study combines ALMA and MUSE observations to investigate the environment of a galaxy group at redshift 0.633, revealing the gas properties, galaxy dynamics, and potential outflows associated with a Ly α absorption system.

Contribution

It presents a detailed multi-wavelength analysis of a galaxy group environment around an HI-absorption system, demonstrating the effectiveness of combined ALMA and MUSE data in studying baryon flows.

Findings

Detected molecular gas and continuum emission from a galaxy at the absorber redshift.

Identified a galaxy group with four members, including the most massive galaxy with CO emission.

The Ly α absorber traces either an outflow or intra-group gas, linking gas dynamics to galaxy properties.

Abstract

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 $z_{\rm abs} = 0.633$, seen towards the quasar J0423$-$0130 ($z_{\rm QSO} = 0.915$). We detect with ALMA the $^{12}$CO(2--1), $^{12}$CO(3--2) and $1.2$~mm continuum emission from a galaxy at the redshift of the Ly $\alpha$ absorber at a projected distance of $135$ 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.