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 redshifts, galaxies in the simulations display larger H-2 mass fractions and lower H-2 depletion time-scales, also in good agreement with observations. The comoving mass density of H-2 in units of the critical density, Omega(H2), peaks at z approximate to 1.2-1.5, later than the predicted peak of the cosmic star formation rate activity, at z approximate to 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-H2 > 10(9)M(circle dot), star formation rates > 10 M-circle dot yr(-1) and stellar masses M-stellar > 10(10) M-circle dot, 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.