Th eG rowth of th eE arliest Supermassiv eB lac k Holes an dT heir Contribution to Reionization

We discuss currently available observational constraints on the reionization history of the intergalacti cm edium (IGM), and the exten tt ow hic ha ccreting blac k holes (BHs) can help explai nt hese observations. We sho wn ew evidence, based on the combined statistics of Lyman α and β absorption in quasar spectra, that the IGM contains as ignifican ta moun to fn eutral hydrogen, and is experiencing rapid ionization at redshift z ∼ 6. However, we argue that quasar BHs, even fain to nes that are belo w the detection thresholds of existing optical surveys, are unlikely to driv et he evolution of the neutral fraction around this epoch, because they would over-produce the present- da ys oft X-ra yb ackground. On the other hand, the seeds of the z ∼ 6q uasar BHs likely appeared at much earlier epochs ( z ∼ 20), and produced hard ionizing radiation by accretion. These early BHs are promising candidates to accoun tf or the high redshift ( z ∼ 15) ionization implied by the recen tc osmic microwave anisotrop yd ata from WMAP .U sing am odel for the growth of BHs by accretion and mergers in ah ierarchica l cosmology ,w es uggest that the early growth of quasars must include as uper-Eddington growth phase, and that, although not ye to ptically identified, the FIRST radio survey ma yh ave already detected several thousand > 10 8 MBHs at z> 6.