MUSE-ALMA Haloes XVI: a census of the condensed baryons in z~0.5 HI absorption-selected galaxies

Abstract

We use the MUSE-ALMA Haloes sample to build a census of the condensed baryons (stars and cold gas) to study the multiphase interstellar medium and circumgalactic medium (CGM). The sample includes 79 gas-rich galaxies at 0.2<z<1.2 located near strong HI Ly-alpha absorbers along quasar sightlines within projected separations of ~5-270 kpc and radial velocity separations of up to 500 km/s. HST spectroscopy characterises the absorbers, while HST imaging, MUSE and ALMA observations trace the stars, ionised gas and molecular gas. We find the following results. (i) HI masses inferred from optical properties show broad agreement with those derived from the CGM absorber properties for impact parameters <100 kpc, although group environments can significantly influence these estimates. (ii) The molecular gas fraction declines more steeply with stellar mass than the HI fraction. This suggests that stars build up more rapidly than the cold gas reservoir, while massive galaxies either consume their molecular gas more efficiently or convert HI into H2 less efficiently. (iii) Galaxies with high stellar surface densities exhibit the highest baryon fractions. This may reflect baryonic feedback, which can produce flatter inner dark matter profiles in high surface density systems, or aperture effects, where the ionised gas kinematic tracer probes only the innermost, baryon-dominated regions in compact sources, but extends further into the dark matter halo in sources with extended baryonic distributions. (iv) A few galaxies are consistent with being the baryon-dominated descendants of the low mass z~6 galaxies detected by JWST, while many appear to be substantially more dark matter dominated, lower mass systems with progenitors beyond JWST detection limits. This study provides a multiscale, multiphase view of the gas in and around galaxies, allowing us to probe the baryon budget across cosmic time.