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

TL;DR

This study models the ionizing photon output of high-redshift galaxies, showing that faint galaxies could sustain reionization by redshift 8, with efficiency increasing at higher redshifts.

Contribution

It introduces a new SED modeling approach incorporating photon escape mechanisms, providing updated ionizing emissivity estimates and insights into galaxy contributions during reionization.

Findings

Faint, blue galaxies are more efficient ionizers than bright, red galaxies.

Galaxies down to M_UV ≈ -15 can sustain reionization by z≈8.

Ionizing efficiency increases with redshift.

Abstract

We present a new analysis of the ionizing emissivity ($\dot{N}_{\rm{ion}}$, s$^{-1}$ Mpc$^{-3}$) for galaxies during the epoch of reionization and their potential for completing and maintaining reionization. We use extensive SED 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 $\dot{N}_{\rm{ion}}$ 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. Firstly, 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 $M_{\rm{UV}} \approx -15$ alone can potentially produce sufficient numbers of ionizing photons to maintain reionization as early as $z\sim8$ for a clumping factor of $C_{\rm{H {\small II}}} \leq 3$.