The Photoevaporation of Dwarf Galaxies during Reionization

During the period of reionization, the universe was filled with a cosmological background of ionizing radiation. By that time a significant fraction of the cosmic gas had already been incorporated into collapsed galactic halos with virial temperatures ≲10⁴ K that were unable to cool efficiently. We show that photoionization of this gas by the fresh cosmic UV background boiled the gas out of the gravitational potential wells of its host halos. We calculate the photoionization heating of gas inside spherically symmetric dark matter halos and assume that gas that is heated above its virial temperature is expelled. In popular cold dark matter models, the Press-Schechter halo abundance implies that ~50%-90% of the collapsed gas was evaporated at reionization. The gas originated from halos below a threshold circular velocity of ~10-15 km s^-1. The resulting outflows from the dwarf galaxy population at redshifts z = 5-10 affected the metallicity and the thermal and hydrodynamic states of the surrounding intergalactic medium. Our results suggest that stellar systems with a velocity dispersion ≲10 km s^-1, such as globular clusters or the dwarf spheroidal galaxies of the Local Group, did not form directly through cosmological collapse at high redshifts.