The Photoevaporation of Dwarf Galaxies during Reionization

During the period of reionization, the universe was lled with a cosmological background of ionizing radiation. By that time a signicant fraction of the cosmic gas had already been incorporated into collapsed galactic halos with virial temperatures K that were unable to cool efficiently. We show that [104 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 D50%90% of the collapsed gas was evaporated at reionization. The gas originated from halos below a threshold circular velocity of D1015 km s~1. The resulting outows from the dwarf galaxy population at redshifts z \ 510 a ected the metallicity and the thermal and hydrodynamic states of the surrounding intergalactic medium. Our results suggest that stellar systems with a velocity dispersion km s~1, such as [10 globular clusters or the dwarf spheroidal galaxies of the Local Group, did not form directly through cosmological collapse at high redshifts.