The Epoch of Reionization
Abstract
We have modelled the process of reionization in the high redshift Universe to determine the epoch of reionization. Reionization is driven by star formation in high redshift galaxies. We employ a semi-analytic model of galaxy formation to track the formation of these galaxies, their influence on the intergalactic medium (IGM) and the back-reaction of the IGM on further galaxy formation. This represents a more complete and physical modelling of reionization than has been conducted in the past. In particular, compared to our previous work our new calculations contain significant improvements in the modelling of the effects of reionization on the galaxy population and in the cooling model used to compute the rate at which galaxies form (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~14-15 in a cosmological model motivated by the results from the Wilkinson Microwave Anisotropy Probe (WMAP) satellite, consistent with the optical depth to reionization measured by WMAP. However, a cosmological model with a running spectral index is able to achieve reionization before z~9 only with very extreme assumptions about the physics of feedback at high redshifts. These results assume that all ionizing photons 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. Under the assumption that early star formation leads to the formation of very massive stars we find that extended periods of partial reionization and double reionizations can occur. Such models do not fully reionize until z~6-7 but obtain an optical depth which is consistent with the results from the WMAP satellite. (abridged)Keywords
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