Molecular hydrogen abundances of galaxies in the EAGLE simulations

We investigate the abundance of galactic molecular hydrogen (H₂) in the ‘Evolution and Assembly of GaLaxies and their Environments’ (EAGLE) cosmological hydrodynamic simulations. We assign H₂ 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 H₂ 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 H₂ and the stellar mass, star formation rate and stellar surface density. Towards high redshifts, galaxies in the simulations display larger H₂ mass fractions and lower H₂ depletion time-scales, also in good agreement with observations. The comoving mass density of H₂ in units of the critical density, ΩH_2⁠, peaks at z ≈ 1.2–1.5, later than the predicted peak of the cosmic star formation rate activity, at z ≈ 2. This difference stems from the decrease in gas metallicity and increase in interstellar radiation field with redshift, both of which hamper H₂ formation. We find that the cosmic H₂ budget is dominated by galaxies with MH₂>10⁹ M_⊙⁠, star formation rates >10 M_⊙ yr⁻¹ and stellar masses M_stellar > 10¹⁰  M_⊙, which are readily observable in the optical and near-IR. The match between the H₂ properties of galaxies that emerge in the simulations and observations is remarkable, particularly since H₂ observations were not used to adjust parameters in EAGLE.