An ionizing UV background (UVB) inhibits gas accretion and photo-evaporates
gas from the shallow potential wells of small, dwarf galaxies. During
cosmological reionization, this effect can result in negative feedback:
suppressing star-formation inside HII regions, thus impeding their continued
growth. It is difficult to model this process, given the enormous range of
scales involved. We tackle this problem using a tiered approach: combining
parameterized results from single-halo collapse simulations with large-scale
models of reionization. In the resulting reionization models, the ionizing
emissivity of galaxies depends on the local values of the reionization redshift
and the UVB intensity. We present a physically-motivated analytic expression
for the average minimum mass of star-forming galaxies, which can be readily
used in modeling galaxy formation. We find that UVB feedback: (i) delays the
end stages of reionization by less than 0.5 in redshift; (ii) results in a more
uniform distribution of HII regions, peaked on smaller-scales (with large-scale
ionization power suppressed by tens of percent); and (iii) suppresses the
global photoionization rate per baryon by a factor of < 2 towards the end of
reionization. However, the impact is modest, since the hydrodynamic response of
the gas to the UVB occurs on a time-scale comparable to reionization. In
particular, the popular approach of modeling UVB feedback with an instantaneous
transition in the minimum mass of star-forming galaxies, dramatically
overestimates its importance. UVB feedback does not significantly affect
reionization unless: (i) molecularly-cooled galaxies contribute significantly
to reionization; or (ii) internal feedback processes strongly couple with UVB
feedback in the early Universe. Since both are considered unlikely, we conclude
that there is no significant self-regulation of reionization by UVB feedback.Comment: 9 pages, 9 figure
