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 \la 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 which is heated above its virial temperature is expelled .
In popular Cold Dark Matter models , the Press-Schechter halo abundance implies that \sim 50 – 90 \% of the collapsed gas was evaporated at reionization .
The gas originated from halos below a threshold circular velocity of \sim 10 –15 km s ^ { -1 } .
The resulting outflows from the dwarf galaxy population at redshifts z = 5 –10 affected the metallicity , thermal and hydrodynamic state of the surrounding intergalactic medium .
Our results suggest that stellar systems with a velocity dispersion \la 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 .