ALMACAL

XIII. Evolution of the CO luminosity function and the molecular gas mass density out to z  ∼  6

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching near Munich, Germany
² Aix Marseille Univ., CNRS, LAM (Laboratoire d’Astrophysique de Marseille), UMR 7326, F-13388 Marseille, France
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Max-Planck-Institut für Extraterrestrische Physik (MPE), Giessenbachstrasse 1, D-85748 Garching, Germany
⁵ Sydney Institute for Astronomy, School of Physics A28, University of Sydney, 2006 NSW, Australia
⁶ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), ACT 2611, Australia
⁷ International Centre for Radio Astronomy Research (ICRAR), M468, University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia
⁸ Cosmic Dawn Center (DAWN), København, Denmark
⁹ Department of Physics & Astronomy, McMaster University, 1280 Main Street W, Hamilton, ON L8S 4M1, Canada
¹⁰ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
¹¹ School of Cosmic Physics, Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin D02 XF86, Ireland
¹² UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK

^⋆ Corresponding author; victoria.bollo@eso.org

Received: 29 November 2024
Accepted: 6 February 2025

Abstract

Cold molecular gas, largely traced by CO emission, is the primary fuel for star formation, making it essential for understanding galaxy evolution. ALMA has made significant progress in the study of the cosmic evolution of cold molecular gas. Here, we exploit the ALMACAL survey to address issues relating to small sample sizes and cosmic variance, utilising calibration data from ALMA to compile a statistically significant and essentially unbiased sample of CO-selected galaxies. By employing a novel statistical approach to emission-line classification using semi-analytical models, we place strong constraints on the CO luminosity function and the cosmic evolution of molecular gas mass density (ρ_H2) back to z ∼ 6. The cosmic molecular gas mass density increases with redshift, peaking around z ∼ 1.5, then slowly declines towards higher redshifts by ∼1 dex. Our findings confirm the key role of molecular gas in fuelling star formation. The new ρ_H2 estimates allow us to revisit the cosmic baryon cycle, showing that the ratio of molecular gas-to-stellar mass density is consistent with the so-called bathtub model of baryons, which implies a continuous replenishment of gas. The cosmic gas depletion timescale, estimated on a global scale, is shown to be fairly constant at all redshifts. We emphasise the importance of surveys using multiple small fields rather than a single contiguous area to mitigate the effects of cosmic variance.