Dark-ages Reionization and Galaxy Formation Simulation - XIV. Gas accretion, cooling and star formation in dwarf galaxies at high redshift.

We study dwarf galaxyformation at high redshift($z\ge5$) using a suite of high- resolution, cosmological hydrodynamic simulations and a semi-analytic model (SAM). We focus on gas accretion, cooling and star formationin this work by isolating the relevant process from reionizationand supernova feedback, which will be further discussed in a companion paper. We apply the SAM to halo merger trees constructed from a collisionless N-body simulationsharing identical initial conditions to the hydrodynamic suite, and calibrate the free parameters against the stellar massfunction predicted by the hydrodynamic simulations at z = 5. By making comparisons of the star formationhistory and gas components calculated by the two modelling techniques, we find that semi-analytic prescriptions that are commonly adopted in the literature of low- redshiftgalaxy formation do not accurately represent dwarf galaxyproperties in the hydrodynamic simulation at earlier times. We propose 3 modifications to SAMs that will provide more accurate high- redshiftsimulations. These include 1) the halo mass and baryon fraction which are overestimated by collisionless N-body simulations; 2) the star formationefficiency which follows a different cosmic evolutionary path from the hydrodynamic simulation; and 3) the cooling rate which is not well defined for dwarf galaxiesat high redshift. Accurate semi-analytic modelling of dwarf galaxyformation informed by detailed hydrodynamical modelling will facilitate reliable semi-analytic predictions over the large volumes needed for the study of reionization.