  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 below about 10000 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 which is heated above its virial temperature
is expelled. In popular Cold Dark Matter models, the Press-Schechter halo
abundance implies that about 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. The resulting outflows from the dwarf galaxy population at
redshifts 5-10 affected the metallicity, thermal and hydrodynamic state of the
surrounding intergalactic medium. Our results suggest that stellar systems with
a velocity dispersion below about 10 km/s, such as globular clusters or the
dwarf spheroidal galaxies of the Local Group, did not form directly through
cosmological collapse at high redshifts.
