Dark-ages reionization and galaxy formation simulation - XV. Stellar evolution and feedback in dwarf galaxies at high redshift

We directly compare predictions of dwarf galaxy properties in a semi-analytic model (SAM) with those extracted from a high-resolution hydrodynamic simulation. We focus on galaxies with halo masses of 10⁹ < M_vir/M⊙ ≲ 10¹¹ at high redshift (z ≥ 5). We find that, with the modifications previously proposed in Qin et al. (2018), including to suppress the halo mass and baryon fraction, as well as to modulate gas cooling and star formation efficiencies, the SAM can reproduce the cosmic evolution of galaxy properties predicted by the hydrodynamic simulation. These include the galaxy stellar mass function, total baryonic mass, star-forming gas mass, and star formation rate at z ~ 5-11. However, this agreement is only possible by introducing a mass-dependent star formation threshold in the SAM, which then prevents excessive star-forming gas from being trapped in quenched dwarf galaxies. As commonly adopted in the literature, thismass-dependent threshold is a phenomenological model based on the observed surface star formation rate to gas density relation of nearby galaxies, and allows one to capture the marginalized process of star formation in high-redshift dwarf galaxies predicted by the hydrodynamic simulation. We further find that dwarf galaxies rapidly build up their star-forming reservoirs in the early universe (z > 10), with the relevant effective timescale becoming significantly longer towards lower redshifts. This indicates efficient accretion in cold mode in these low-mass objects at high redshift. The improved SAM, which has been calibrated against hydrodynamic simulations, can provide more accurate predictions of high-redshift dwarf galaxy properties that are essential for reionization study.