Trends in Large Scale Structure Observations and the Likelihood of Early Reionization

With the imminent promise of constraints on the epoch of reionization from observations of microwave background anisotropies, the question of whether or not the standard Cold Dark Matter (CDM) model permits early reionization has been subjected to detailed investigation by various authors, with the conclusion that reionization may occur at quite high redshift. However, it is widely accepted that this model is excluded, as when normalised to the COBE observations it possesses excessive galaxy clustering on scales below tens of megaparsecs. We examine the trends of observations, rst in a fairly model independent way, and second by considering variants on the standard CDM model introduced to resolve the observational connicts. We conclude that the epoch of reionization favoured by the observational data is typically considerably later than the standard CDM model suggests, and amongst models which may t the observational data only the introduction of a cosmological constant leads to a reionization redshift close to that of standard CDM.