Molecular hydrogen abundances of galaxies in the EAGLE simulations
  We investigate the abundance of galactic molecular hydrogen (H$_2$) in the
"Evolution and Assembly of GaLaxies and their Environments" (EAGLE)
cosmological hydrodynamic simulations. We assign H$_2$ masses to gas particles
in the simulations in post-processing using two different prescriptions that
depend on the local dust-to-gas ratio and the interstellar radiation field.
Both result in H$_2$ galaxy mass functions that agree well with observations in
the local and high-redshift Universe. The simulations reproduce the observed
scaling relations between the mass of H$_2$ and the stellar mass, star
formation rate and stellar surface density. Towards high edshifts, galaxies in
the simulations display larger H$_2$ mass fractions, and correspondingly lower
H$_2$ depletion timescales, also in good agreement with observations. The
comoving mass density of H$_2$ in units of the critical density, $\Omega_{\rm
H_2}$, peaks at $z\approx 1.2-1.5$, later than the predicted peak of the cosmic
star formation rate activity, at $z\approx 2$. This difference stems from the
decrease in gas metallicity and increase in interstellar radiation field with
redshift, both of which hamper H$_2$ formation. We find that the cosmic H$_2$
budget is dominated by galaxies with $M_{\rm H_2}>10^9\,\rm M_{\odot}$, star
formation rates $>10\,\rm M_{\odot}\,\rm yr^{-1}$ and stellar masses $M_{\rm
stellar}>10^{10}\,\rm M_{\odot}$, which are readily observable in the optical
and near-IR. The match between the H$_2$ properties of galaxies that emerge in
the simulations and observations is remarkable, particularly since H$_2$
observations were not used to adjust parameters in EAGLE.
