Early Reionization by Miniquasars

Motivated by the recent detection by WMAP of a large optical depth to Thomsonscattering -- implying a very early reionization epoch -- we assess a scenariowhere the universe was reionized by `miniquasars' powered by intermediate-massblack holes (IMBHs), the remnants of the first generation of massive stars.Pregalactic IMBHs form within minihalos above the cosmological Jeans masscollapsing at z=24, get incorporated through mergers into larger and largersystems, sink to the center owing to dynamical friction, and accrete coldmaterial. The merger history of dark halos and associated IMBHs is followed byMonte Carlo realizations of the merger hierarchy in a LCDM cosmology. Whileseed IMBHs that are as rare as the 3.5-sigma peaks of the primordial densityfield evolve largely in isolation, a significant number of black hole binarysystems will form if IMBHs populate the more numerous 3-sigma peaks instead. Inthe case of rapid binary coalescence a fraction of IMBHs will be displaced fromgalaxy centers and ejected into the IGM by the `gravitational rocket' effect,rather than accrete and shine as miniquasars. We show that, under a number ofplausible assumptions for the amount of gas accreted onto IMBHs and theiremission spectrum, miniquasars powered by IMBHs may be responsible forcosmological reionization at z~15. Reionization by miniquasars with a hardspectrum may be more `economical' than stellar reionization, as soft X-raysescape more easily from the dense sites of star formation and travel furtherthan EUV radiation. Energetic photons will make the diffuse IGM warm and weaklyionized prior to the epoch of reionization breakthrough, set an entropy floor,and reduce gas clumping. Future 21 cm observations may detect a preheated,weakly-ionized IGM in emission against the CMB. (abridged)Comment: 12 pages, 8 figures, revised version accepted for publication in the Astrophysical Journa