Molecular hydrogen abundances of galaxies in the EAGLE simulations

Abstract

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, ${\Omega }_{{\mathrm{H}}_2}$, 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 $M_{{\mathrm{H}}_2}>10^9{\mathrm{M}}_{\odot }$, star formation rates $>10{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$ and stellar masses $M_{\mathrm{stellar}}>10^{10}{\mathrm{M}}_{\odot }$, 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.

24 pages and 21 figures (18 pages and 16 figures without appendices). Accepted for publication in MNRAS

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