  One of the largest uncertainties in understanding the effect of a background
UV field on galaxy formation is the intensity and evolution of the radiation
field with redshift. This work attempts to shed light on this issue by
computing the quasi-hydrostatic equilibrium states of gas in spherically
symmetric dark matter halos (roughly corresponding to dwarf galaxies) as a
function of the amplitude of the background UV field. We integrate the full
equations of radiative transfer, heating, cooling and non-equilibrium chemistry
for nine species: H, H^+, H^-,H_2, H_2^+, He, He^+, He^{++}, and e^-. As the
amplitude of the UV background is decreased the gas in the core of the dwarf
goes through three stages characterized by the predominance of ionized (H^+),
neutral (H) and molecular (H_2) hydrogen. Characterizing the gas state of a
dwarf galaxy with the radiation field allows us to estimate its behavior for a
variety of models of the background UV flux. Our results indicate that a
typical radiation field can easily delay the collapse of gas in halos
corresponding to 1-$\sigma$ CDM perturbations with circular velocities less
than 30 km/s.
