Automated Mining of the ALMA Archive in the COSMOS Field (A3COSMOS). II. Cold Molecular Gas Evolution out to Redshift 6

Abstract

We present new measurements of the cosmic cold molecular gas evolution out to redshift 6 based on systematic mining of the Atacama Large Millimeter/submillimeter Array (ALMA) public archive in the COSMOS deep field (A3COSMOS). Our A3COSMOS data set contains ~700 galaxies (0.3 lesssim z lesssim 6) with high-confidence ALMA detections in the (sub)millimeter continuum and multiwavelength spectral energy distributions. Multiple gas mass calibration methods are compared, and biases in band conversions (from observed ALMA wavelength to rest-frame Rayleigh-Jeans tail continuum) have been tested. Combining our A3COSMOS sample with ~1000 CO-observed galaxies at 0 lesssim z lesssim 4 (75% at z < 0.1), we parameterize galaxies' molecular gas depletion time (${\tau }_{\mathrm{depl}}$) and molecular gas to stellar mass ratio (${\mu }_{\mathrm{molgas}}$) each as a function of the stellar mass (${M}_{\star }$), offset from the star-forming main sequence (${\rm{\Delta }}\mathrm{MS}$) and cosmic age (or redshift). Our proposed functional form provides a statistically better fit to current data (than functional forms in the literature) and implies a "downsizing" effect (i.e., more-massive galaxies evolve earlier than less-massive ones) and "mass quenching" (gas consumption slows down with cosmic time for massive galaxies but speeds up for low-mass ones). Adopting galaxy stellar mass functions and applying our ${\mu }_{\mathrm{molgas}}$ function for gas mass calculation, we for the first time infer the cosmic cold molecular gas density evolution out to redshift 6 and find agreement with CO blind surveys as well as semianalytic modeling. These together provide a coherent picture of cold molecular gas, star formation rate, and stellar mass evolution in galaxies across cosmic time.