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2026A&A...707A..81B - Astronomy and Astrophysics, volume 707A81 (2026/3-1)

MUSE-ALMA Haloes XII. Molecular gas in z ∼ 0.5 H I - selected galaxies.

BOLLO V., PEROUX C., ZWAAN M., CHEN J., KULKARNI V.P., BARFETY C., WENG S., FORSTER SCHREIBER N.M., TACCONI L., CASAVECCHIA B., O'BEIRNE T., CHEMIN L., AUGUSTIN R. and HALLEY M.

Abstract (from CDS):

We present further results from the MUSE-ALMA Haloes survey, which includes 79 galaxies associated with strong H I absorption at z ∼ 0.5. As part of this effort, our ALMA Cycle 10 Large Program contributed new observations of 39 systems. This expands on the initial set of 21 systems in the MUSE-ALMA Haloes survey, bringing the total to 60 galaxies. Among the newly observed systems, we detect CO line emission in nine galaxies, corresponding to a 23% detection rate in a sample not selected by metallicity. When combined with prior MUSE-ALMA Haloes data, our total CO detection count rises to 12 out of 60 galaxies (20%), which effectively doubles the number of detected CO-emitting H I-selected galaxies at z ∼ 0.5 and probes a factor of ∼1.2 dex deeper in MH2 than earlier absorber studies. These sources, which were selected based on known circumgalactic H I gas, span a wide range of stellar masses and metallicities, providing a unique view of gas-rich environments. By comparing the molecular gas properties, traced through CO(2-1) and CO(3-2) transitions with existing information of their physical properties, such as star formation rates (SFRs) and gas-phase metallicities from VLT/MUSE and HST spectroscopy, we investigate how these systems relate to the population of normal star-forming galaxies at similar redshift. Our deep, unbiased CO observations of H I-selected galaxies reveal a dual behaviour in star formation efficiency. Low-MH2 systems form stars efficiently and follow the scaling relations of main-sequence galaxies, while high-MH2 systems exhibit suppressed star formation and lower-than-expected stellar masses, likely reflecting ongoing gas accretion or environmental regulation. This diversity indicates that H I absorbers trace both evolved, actively star-forming galaxies and younger or dynamically influenced systems that are still building their gas reservoirs. By reaching molecular gas masses more than 1 dex below previous studies, our survey provides a key step towards completing the baryon census at z ∼ 0.5 and characterising the molecular phase of the broader H I-selected population.

Abstract Copyright: © The Authors 2026

Journal keyword(s): galaxies: evolution - intergalactic medium - quasars: absorption lines - galaxies: star formation

Simbad objects: 8

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