How does radiative feedback from an ultraviolet background impact reionization?

An ionizing ultraviolet background (UVB) inhibits gas accretion and photoevaporates gas from the shallow potential wells of small, dwarf galaxies. During cosmological reionization, this effect can result in negative feedback: suppressing star formation inside H ii 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 parametrized 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, (M) over bar (min), which can be readily used in modelling galaxy formation. We find that UVB feedback: (i) delays the end stages of reionization by delta z less than or similar to 0.5; (ii) results in a more uniform distribution of H ii regions, peaked on smaller scales (with large-scale ionization power suppressed by 10s of per cent) and (iii) suppresses the global photoionization rate per baryon by a factor of less than or similar to 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 modelling UVB feedback with an instantaneous transition in M-min, dramatically overestimates its importance. UVB feedback on galaxies 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.