  CO measurements of z~1-4 galaxies have found that their baryonic gas
fractions are significantly higher than galaxies at z=0, with values ranging
from 20-80 %. 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 (Xco). Evidence from
both observations and numerical models suggest that Xco varies smoothly with
the physical properties of galaxies, and that Xco can be parameterised simply
as a function of both gas phase metallicity and observed CO surface brightness.
When applying this functional form, we find fgas ~10-40 % in galaxies with
M*=10^10-10^12 Msun at high-z. Moreover, the scatter in the observed fgas-M*
relation is lowered by a factor of two. 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 Xco 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 Xco values for z=1-4
galaxies results in revised gas fractions that agree significantly better with
the simulations.
