The Photoevaporation of Dwarf Galaxies during Reionization

During the period of reionization the Universe was filled with a cosmologicalbackground of ionizing radiation. By that time a significant fraction of thecosmic gas had already been incorporated into collapsed galactic halos withvirial temperatures below about 10000 K that were unable to cool efficiently.We show that photoionization of this gas by the fresh cosmic UV backgroundboiled the gas out of the gravitational potential wells of its host halos. Wecalculate the photoionization heating of gas inside spherically symmetric darkmatter halos, and assume that gas which is heated above its virial temperatureis expelled. In popular Cold Dark Matter models, the Press-Schechter haloabundance implies that about 50-90% of the collapsed gas was evaporated atreionization. The gas originated from halos below a threshold circular velocityof 10-15 km/s. The resulting outflows from the dwarf galaxy population atredshifts 5-10 affected the metallicity, thermal and hydrodynamic state of thesurrounding intergalactic medium. Our results suggest that stellar systems witha velocity dispersion below about 10 km/s, such as globular clusters or thedwarf spheroidal galaxies of the Local Group, did not form directly throughcosmological collapse at high redshifts.Comment: 29 pages, 7 PostScript figures, accepted for ApJ. Final version, revised due to referee comments. Figures 6 & 7 have been corrected for a small numerical erro