MUSE-ALMA Haloes XIII. Molecular gas in $z \sim 0.5$ HI-selected galaxies

TL;DR

This study investigates molecular gas in 60 HI-selected galaxies at redshift 0.5, revealing diverse star formation efficiencies and expanding the understanding of galaxy evolution through new CO detections.

Contribution

It provides the largest sample of CO-emitting HI-selected galaxies at z~0.5, with deeper molecular gas mass probing and analysis of star formation efficiency variations.

Findings

Doubling the number of CO-detected HI-selected galaxies.

Detection of a dual behavior in star formation efficiency.

Galaxies span a wide range of stellar masses and metallicities.

Abstract

We present new results from the MUSE-ALMA Haloes survey, covering 79 galaxies associated with strong HI absorption at redshift about 0.5. Our ALMA Cycle 10 observations add 39 systems to the initial 21, bringing the total to 60 galaxies. CO emission is detected in 9 new galaxies, and in 12 of 60 total, doubling the number of CO-emitting HI-selected galaxies and probing 1.2 dex deeper in molecular gas mass than previous studies. These galaxies span a wide range of stellar masses and metallicities. By comparing CO(2-1) and CO(3-2) properties with star formation rates and gas-phase metallicities from VLT/MUSE and HST, we find a dual behaviour in star formation efficiency: low-mass systems follow main-sequence scaling relations, while high-mass systems show suppressed star formation. This diversity indicates that HI absorbers trace both evolved and younger galaxies, providing a key step toward completing the baryon census at redshift about 0.5.