Evolution of the atomic and molecular gas content of galaxies in dark matter haloes

We present a semi-empirical model to infer the atomic and molecular hydrogen content of galaxies as a function of halo mass and time. Our model combines the star formation rate (SFR)-halo mass-redshift relation (constrained by galaxy abundances) with inverted SFR- surface density relations to infer galaxy HI and H₂ masses. We present gas scaling relations, gas fractions, and mass functions from z = 0 to 3 and the gas properties of galaxies as a function of their host halo masses. Predictions of our work include: (1) there is an ~0.2 dex decrease in the HI mass of galaxies as a function of their stellar mass since z=1.5, whereas the H₂ mass of galaxies decreases by >1 dex over the same period; (2) galaxy cold gas fractions and H₂ fractions decrease with increasing stellarmass and time. Galaxies with M* > 10¹⁰M⊙ are dominated by their stellar content at z ≤ 1, whereas less-massive galaxies only reach these gas fractions at z = 0. We find the strongest evolution in relative gas content at z < 1.5; (3) the SFR to gas mass ratio decreases by an order of magnitude from z = 3 to 0. This is accompanied by lower H₂ fractions; these lower fractions in combination with smaller gas reservoirs correspond to decreased present-day galaxy SFRs; (4) an H₂-based star formation relation can simultaneously fuel the evolution of the cosmic star formation and reproduce the observed weak evolution in the cosmic HI density; (5) galaxies residing in haloes with masses near 10¹²M⊙ are most efficient at obtaining large gas reservoirs and forming H₂ at all redshifts. These two effects lie at the origin of the high star formation efficiencies in haloes with the same mass.