Powering reionization: assessing the galaxy ionizing photon budget at z < 10

We present a new analysis of the ionizing emissivity ((N) over dot(ion), s(-1) Mpc(-3)) for galaxies during the epoch of reionization and their potential for completing and maintaining reionization. We use extensive spectral energy distribution modelling - incorporating two plausible mechanisms for the escape of Lyman continuum photon - to explore the range and evolution of ionizing efficiencies consistent with new results on galaxy colours (beta) during this epoch. We estimate (N) over dot(ion) for the latest observations of the luminosity and star formation rate density at z < 10, outlining the range of emissivity histories consistent with our new model. Given the growing observational evidence for a UV colour-magnitude relation in high-redshift galaxies, we find that for any plausible evolution in galaxy properties, red (brighter) galaxies are less efficient at producing ionizing photons than their blue (fainter) counterparts. The assumption of a redshift and luminosity evolution in beta leads to two important conclusions. First, the ionizing efficiency of galaxies naturally increases with redshift. Secondly, for a luminosity-dependent ionizing efficiency, we find that galaxies down to a rest-frame magnitude of M-UV approximate to -15 alone can potentially produce sufficient numbers of ionizing photons to maintain reionization as early as z similar to 8 for a clumping factor of C-H II <= 3.