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

Abstract

CO measurements of z~1-4 galaxies have found that their baryonic gas\nfractions are significantly higher than galaxies at z=0, with values ranging\nfrom 20-80 %. Here, we suggest that the gas fractions inferred from\nobservations of star-forming galaxies at high-z are overestimated, owing to the\nadoption of locally-calibrated CO-H2 conversion factors (Xco). Evidence from\nboth observations and numerical models suggest that Xco varies smoothly with\nthe physical properties of galaxies, and that Xco can be parameterised simply\nas a function of both gas phase metallicity and observed CO surface brightness.\nWhen applying this functional form, we find fgas ~10-40 % in galaxies with\nM*=10^10-10^12 Msun at high-z. Moreover, the scatter in the observed fgas-M*\nrelation is lowered by a factor of two. The lower inferred gas fractions arise\nphysically because the interstellar media of high-z galaxies have higher\nvelocity dispersions and gas temperatures than their local counterparts, which\nresults in an Xco that is lower than the z=0 value for both quiescent discs and\nstarbursts. We further compare these gas fractions to those predicted by\ncosmological galaxy formation models. We show that while the canonically\ninferred gas fractions from observations are a factor of 2-3 larger at a given\nstellar mass than predicted by models, our rederived Xco values for z=1-4\ngalaxies results in revised gas fractions that agree significantly better with\nthe simulations.\n