Modelling the escape of Lyman Continuum photons from galaxies in the Epoch of Reionization

Abstract

We couple the DELPHI framework for galaxy formation with a model for the escape of ionizing photons to study both its variability with galaxy assembly and the resulting key reionization sources. In this model, leakage either occurs through a fully ionized gas distribution (ionization bounded) or additionally through channels cleared of gas by supernova explosions (ionization bounded + holes). The escape fraction is therefore governed by a combination of the density and star formation rate. Having calibrated our star formation efficiencies to match high-$z$ observables, we find the central gas density to regulate the boundary between high ($>0.70$) and low ($<0.06$) escape fractions. As galaxies become denser at higher redshifts, this boundary shifts from $M_{h}\simeq 10^{9.5}\mathrm{M_{\odot}}$ at $z\sim 5$ to $M_{h}\simeq 10^{7.8}\mathrm{M_{\odot}}$ at $z\sim 15$. While leakage is entirely governed through holes above this mass range, it is not affecting general trends for lower masses. We find the co-evolution of galaxy assembly and the degree of leakage to be mass and redshift dependent, driven by an increasing fraction of $f_{\mathrm{esc}}<0.06$ galaxies at increasing mass and redshift. The variability in the escape of ionizing photons is driven by the underlying variations in our dark matter assembly histories. Galaxies with $M_h < 10^{7.9} ~ (10^{8.9})M_{\odot}$ provide half of the escaping ionizing emissivity by $z \sim 10 ~ (5)$ in the ionization bounded model. On the other hand, galaxies that purely leak through holes contribute $6$ $(13)\%$ at $z\sim 5$ $(15)$. We end by exploring the impact of two reionization feedback scenarios, in which we suppress the gas content of galaxies with $T_{\mathrm{vir}}<20000\mathrm{K}$ and $v_{c}<30\mathrm{kms^{-1}}$ residing in ionized regions.