UV Radiative Feedback During the Advanced Stages of Reionization

The ionizing ultraviolet background (UVB) during reionization can suppress the gas content of low-mass galaxies, even those capable of efficient atomic cooling, and thus lead to an extended reionization epoch. In this work, we explore the importance of negative UV radiative feedback on Tvir > 10^4 K halos during the middle and late stages of reionization. We do not try to self-consistently model reionization; instead, we explore a large parameter space in an attempt to draw general, robust conclusions. We do this using a tiered approach. Using 1-D hydrodynamical simulations, we model the collapse of gas onto halos of various masses under UVBs of various intensities. We then generate realistic, parametrized maps of the inhomogeneous UVB, using large-scale semi-numeric simulations. By combining these results, we find that under all reasonably conservative scenarios, UV feedback on atomically-cooled halos is not strong enough to notably delay the bulk of reionization. Such a delay is only likely if ionizing efficiencies of z > 10 sources are much higher (~ two orders of magnitude) than z ~ 6 data seem to imply. We also find that feedback is very strongly dependent on halo mass. Our results suggest that the natural time-scale for the bulk of reionization is the growth of the global collapsed fraction contained in Tvir > 10^4 K halos. Finally, our results underscore the importance of taking into account extended dynamical ranges when modeling reionization.