Probing the reionization history of the Universe

We discuss currently available observational constraints on the reionization history of the intergalactic medium (IGM), and the extent to which accreting black holes (BHs) and stars can help account for these observations. We argue, based on the combined statistics of Lyman α and β absorption in quasar spectra, that the IGM contains a significant amount of neutral hydrogen with nH I/nH ≳ 0.1. On the other hand, we argue, based on the lack of a strong evolution in the observed abundance of Lyman α emitting galaxies beyond z ∼ 5.5, that the mean neutral hydrogen fraction cannot exceed nH I/nH ≈ 0.3 at the same redshift. We conclude that the IGM is experiencing rapid ionization at redshift z ∼ 6. We find that quasar BHs, including faint ones that are individually below the detection thresholds of existing optical and X-ray surveys, are unlikely to drive the evolution of the neutral fraction around this epoch, because they would over-produce the present-day soft X-ray background. On the other hand, the seeds of the z ∼ 6 quasar BHs likely appeared at much earlier epochs (z ∼ 20), and produced hard ionizing radiation by accretion. These early BHs are promising candidates to account for the high redshift (z ∼ 15) ionization implied by the recent cosmic microwave anisotropy data from WMAP. Using a model for the growth of BHs by accretion and mergers in a hierarchical cosmology, we suggest that the early growth of quasars must include a super-Eddington growth phase, and that, although not yet optically identified, the FIRST radio survey may have already detected several thousand > 10^8 M⊙ BHs at z > 6.