The Photoevaporation of Dwarf Galaxies during Reionization

Abstract

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 less than or similar to 10(4) 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 similar to 50%-90% of the collapsed gas was evaporated at reionization. The gas originated from halos below a threshold circular velocity of similar to 10-15 km s(-1). The resulting outflows front 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 less than or similar to 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.