The epoch of reionization

Abstract

We have modelled the process of reionization of the intergalactic medium (IGM) by photoionization by galaxies, in order to learn what galaxy formation in the framework of the cold dark matter (CDM) model predicts for the epoch of reionization. We use a sophisticated semi-analytic model of galaxy formation to track the formation of these galaxies, their influence on the IGM and the back reaction of the state of the IGM on further galaxy formation. Our study represents a much more complete and physically consistent modelling of reionization than has been conducted in the past. In particular, compared to previous work by ourselves and others, our new calculations contain significant improvements in the modelling of the effects of reionization of the IGM on the collapse of baryons into dark matter haloes (this is now computed self-consistently from the properties of model galaxies), and in the model for the cooling and condensation of gas within haloes (our new model includes photoheating from a self-consistently computed ionizing background and also includes cooling due to molecular hydrogen). We find that reionization can be achieved by z∼ 10–20 in a ΛCDM cosmological model with σ8≈ 0.9. However, a cosmological model with a running spectral index is only able to achieve reionization before z≈ 9, and thus be consistent with an optical depth of 0.1, if very extreme assumptions are made about the physics of feedback at high redshifts. We also consider the specific galaxy formation model recently discussed by Baugh et al., which includes a top-heavy initial mass function (IMF) in starbursts, 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 per cent. Such models do not fully reionize until z≈ 6–7, but predict an electron scattering optical depth as large as 0.15. The recent Wilkinson Microwave Anisotropy Probe (WMAP) three-year data suggest lower values of σ8= 0.7–0.8. Models with lower σ8 have reduced redshifts of reionization, consistent with the lower optical depth also suggested by the WMAP three-year data.