  The cosmic ultraviolet (UV) ionizing background impacts the formation of
dwarf galaxies in the low-redshift universe (z=3) by suppressing gas infall
into galactic halos with circular velocities up to v(circ)=75 km/s. Using a
one-dimensional, spherically symmetric hydrodynamics code (Thoul & Weinberg
1995), we examine the effect of an ionizing background on low-mass galaxies
forming at high redshifts (z>10). We find that the importance of
photoionization feedback is greatly reduced, because (1) at high redshift,
dwarf-galaxy sized objects can self-shield against the ionizing background, (2)
collisional cooling processes at high redshift are more efficient, (3) the
amplitude of the ionizing background at high redshift is lower, and (4) the
ionizing radiation turns on when the perturbation that will become the dwarf
galaxy has already grown to a substantial overdensity. We find that because of
these reasons, gas can collect inside halos with circular velocities as low as
v(circ)=10 km/s. This result has important implications for the reionization
history of the universe.
