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

We present a new analysis of the ionizing emissivity (⁠N˙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 (β) during this epoch. We estimate N˙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 β 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 MUV ≈ −15 alone can potentially produce sufficient numbers of ionizing photons to maintain reionization as early as z ∼ 8 for a clumping factor of CHII≤3⁠.