A Limit from the X-ray Background on the Contribution of Quasars to   Reionization
  A population of black holes (BHs) at high redshifts (z>6) that contributes significantly to the ionization of the intergalactic medium (IGM) would be accompanied by the copious production of hard (>10keV) X-ray photons. The resulting hard X-ray background would redshift and be observed as a present-day soft X-ray background (SXB). Under the hypothesis that BHs are the main producers of reionizing photons in the high-redshift universe, we calculate their contribution to the present-day SXB. Our results, when compared to the unresolved component of the SXB in the range 0.5-2 keV, suggest that accreting BHs (be it luminous quasars or their lower-mass ``miniquasar'' counterparts) did not dominate reionization. Distant miniquasars that produce enough X-rays to only partially ionize the IGM to a level of at most 50% are still allowed, but could be severely constrained by improved determinations of the unresolved component of the SXB. 
Mark Dijkstra, Zoltan Haiman (Columbia), Abraham Loeb (Harvard)