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We discuss currently available observational constraints on the reionizationhistory of the intergalactic medium (IGM), and the extent to which accretingblack holes (BHs) can help explain these observations. We show new evidence,based on the combined statistics of Lyman alpha and beta absorption in quasarspectra, that the IGM contains a significant amount of neutral hydrogen, and isexperiencing rapid ionization around redshift z=6. However, we argue thatquasar BHs, even faint ones that are below the detection thresholds of existingoptical surveys, are unlikely to drive the evolution of the neutral fractionaround this epoch, because they would over-produce the present-day soft X-raybackground. On the other hand, the seeds of the quasar BHs around z=6 likelyappeared at much earlier epochs (around z=20), and produced hard ionizingradiation by accretion. These early BHs are promising candidates to account forthe high redshift ionization (around z=15) implied by the recent cosmicmicrowave anisotropy data from WMAP. Using a model for the growth of BHs byaccretion and mergers in a hierarchical cosmology, we suggest that the earlygrowth of quasars must include a super-Eddington growth phase, and that,although not yet optically identified, the FIRST radio survey may have alreadydetected several thousand BHs at z>6 with Mbh>10^8 Msun.Comment: 12 pages, invited contribution to Proceedings of the Conference on  "Growing Black Holes" held in Garching, Germany, on June 21-25, 2004, edited  by A. Merloni, S. Nayakshin and R. Sunyaev, Springer-Verlag series of "ESO  Astrophysics Symposia

The Growth of the Earliest Supermassive Black Holes and Their Contribution to Reionization