The Epoch of Reionization

Abstract

We have modelled the process of reionization of the IGM by photoionization by galaxies, in order to predict the epoch of reionization. We use a sophisticated semi-analytic model to track the formation of galaxies. Our study represents a much more complete and physically consistent modelling of reionization than has been conducted in the past, containing significant improvements in the modelling of the collapse of baryons into dark matter halos, and in the model for the cooling and condensation of gas within halos (including photoheating from a self-consistently computed ionizing background and cooling due to H2). We find that reionization can be achieved by z~10-20 in a LCDM cosmological model with sigma8~0.9. However, a cosmological model with a running spectral index is only able to achieve reionization before z~9 if extreme assumptions are made about the physics of feedback at high redshifts. We also consider the galaxy formation model discussed by Baugh et al., and find that it is able to reionize the Universe by z~12. The previous results assume that all of the ionizing photons produced by stars in galaxies are able to escape and ionize the IGM. If this is not the case, then the redshift of reionization could be substantially reduced. We find that extended periods of partial reionization and double reionizations can occur in models in which the first stars formed via cooling by H2 molecules, are very massive, and in which the escape fraction of ionizing photons ~10-30%. Such models do not fully reionize until z~6-7, but predict an electron scattering optical depth as large as 0.15. Models with lower sigma8=0.7-0.8 as suggested by the recent WMAP three year data have reduced redshifts of reionization, but can be consistent with the lower optical depth also suggested by the WMAP three year data.