Galaxy gas fractions at high redshift: The tension between observations and cosmological simulations

CO measurements of z ∼ 1-4 galaxies have found that their baryonic gas fractions are significantly higher than those for galaxies at z = 0, with values ranging from 20 to 80 per cent. Here, we suggest that the gas fractions inferred from observations of star-forming galaxies at high-z are overestimated, owing to the adoption of locally calibrated CO-H₂ conversion factors (α_CO). Evidence from both observations and numerical models suggests that α_CO varies smoothly with the physical properties of galaxies, and that α_CO can be parametrized simply as a function of both gas-phase metallicity and observed CO surface brightness. When applying this functional form, we find f_gas ≈ 10-40 per cent in galaxies with M_* = 10¹⁰-10¹²M_⊙. Moreover, the scatter in the observed f_gas-M_* relation is lowered by a factor of 2. The lower inferred gas fractions arise physically because the interstellar media of high-z galaxies have higher velocity dispersions and gas temperatures than their local counterparts, which results in an α_CO that is lower than the z = 0 value for both quiescent discs and starbursts. We further compare these gas fractions to those predicted by cosmological galaxy formation models. We show that while the canonically inferred gas fractions from observations are a factor of 2-3 larger at a given stellar mass than predicted by models, our rederived α_CO values for z = 1-4 galaxies result in revised gas fractions that agree significantly better with the simulations.