Effect of ionizing photon escape fraction in faint galaxies on modeling reionization history of the universe

We present model calculations of the reionization history of hydrogen using star formation histories, computed with a galaxy formation model which reproduces properties of local dwarf galaxies and UV luminosity functions of galaxies at \(z=5-16\). We use the ionizing photon density functions predicted by the model along with different models for the escape fraction of ionizing photons, \(f_{\rm esc}\), to study the effects of ionizing photons from faint galaxies and different assumptions about \(f_{\rm esc}\) on the evolution of hydrogen ionized fraction with redshift, \(Q_{\rm HII}(z)\). We show that accounting for the contribution of faint galaxies with UV luminosities \(M_{1500}>-13\), and with a {\it constant} ionizing photon escape fraction of \(f_{\rm esc}=0.1\) results in the hydrogen reionization history consistent with all current observational constraints. Comparing results of the \(f_{\rm esc}=0.1\) model and two alternative models shows that the model with a strong luminosity dependence of \(f_{\rm esc}\), which assigns high \(f_{\rm esc}\) to faint galaxies, results in early reionization inconsistent with observational constraints. However, the model in which \(f_{\rm esc}\) follows a universal redshift-independent correlation with the recent maximum specific star formation rate, motivated by the results of the SPHINX galaxy formation simulation, results in the reionization history in good agreement with existing observational constraints, even though this model produces a sizeable ionized hydrogen fraction of \(Q_{\rm HII}\approx 0.15-0.2\) at redshifts \(z=8-12\).
Our results show that the relative contribution of faint dwarf galaxies to reionization depends sensitively on assumptions about the escape fraction for galaxies of different luminosities, and that this is the main source of uncertainty in modeling hydrogen reionization.