H_2 molecular gas absorption-selected systems trace CO molecular gas-rich galaxy overdensities

TL;DR

This study uses ALMA to observe CO molecular gas in galaxies associated with H_2 absorbers, revealing that such gas often resides in dense circum-galactic regions and multiple galaxy systems, challenging previous assumptions about galaxy environments.

Contribution

First ALMA survey of CO in H_2 absorption-selected galaxies showing dense gas in circum-galactic and intra-group media, often involving multiple galaxies.

Findings

Majority of absorbers linked to multiple galaxies

H_2 gas not confined to galaxy disks

Dense gas pockets in circum-galactic medium

Abstract

Absorption-selected galaxies offer an effective way to study low-mass galaxies at high redshift. However, the physical properties of the underlying galaxy population remains uncertain. In particular, the multiphase circum-galactic medium is thought to hold key information on gas flows into and out of galaxies that are vital for galaxy evolution models. Here we present ALMA observations of CO molecular gas in host galaxies of H_2-bearing absorbers. In our sample of six absorbers we detect molecular gas-rich galaxies in five absorber fields although we did not target high-metallicity (>50 per cent solar) systems for which previous studies reported the highest detection rate. Surprisingly, we find that the majority of the absorbers are associated with multiple galaxies rather than single haloes. Together with the large impact parameters these results suggest that the H_2-bearing gas seen in absorption is not part of an extended disk, but resides in dense gas pockets in the circum-galactic and intra-group medium.