Simulating the H content of high-redshift galaxies

We introduce a sub-grid model for the non-equilibrium abundance of molecular hydrogen in cosmological simulations of galaxy formation.We improve upon previous work by accounting for the unresolved structure of molecular clouds in a phenomenological way which combines both observational and numerical results on the properties of the turbulent interstellar medium. We apply the model to a cosmological simulation of the formation of a Milky Way-sized galaxy at z = 2, and compare the results to those obtained using other popular prescriptions that compute the equilibrium abundance of H. In these runs, we introduce an explicit link between star formation and the local H abundance, and perform an additional simulation in which star formation is linked directly to the density of cold gas. In better agreement with observations, we find that the simulated galaxy produces less stars and harbours a larger gas reservoir when star formation is regulated by molecular hydrogen. In this case, the galaxy is composed of a younger stellar population as early star formation is inhibited in small, metal-poor dark-matter haloes which cannot efficiently produce H. The number of luminous satellites orbiting within the virial radius of the galaxy at z = 2 is reduced by 10-30 per cent in models with H-regulated star formation.