A suppressed contribution of low-mass galaxies to reionization due to supernova feedback

Motivated by recent observations of the star formation rate density function out to z similar to 7, we describe a simple model for the star formation rate density function at high redshift based on the extended Press-Schechter formalism. This model postulates a starburst following each major merger, lasting for a time t(SF) and converting at most f(star,max) of galactic gas into stars. We include a simple physical prescription for supernova feedback that suppresses star formation in low-mass galaxies. Constraining tSF and f(star,max) to describe the observed star formation rate density at high redshifts, we find that individual starbursts were terminated after a time of t(SF) similar to 10(7) yr. This is comparable to the main-sequence lifetimes of supernova progenitors, indicating that high-redshift starbursts are quenched once supernova feedback had time to develop. High-redshift galaxies convert similar to 10 per cent of their mass into stars for galaxies with star formation rates above similar to 1 M-circle dot yr(-1), but a smaller fraction for lower luminosity galaxies. Our best-fitting model successfully predicts the observed relation between star formation rate and stellar mass at z greater than or similar to 4, while our deduced relation between stellar mass and halo mass is also consistent with data on the dwarf satellites of the Milky Way. We find that supernova feedback lowers the efficiency of star formation in the lowest mass galaxies and makes their contribution to reionization small. As a result, photoionization feedback on low-mass galaxy formation does not significantly affect the reionization history. Using a semi-analytic model for the reionization history, we infer that approximately half of the ionizing photons needed to complete reionization have already been observed in star-forming galaxies.