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 contributessignificantly to the ionization of the intergalactic medium (IGM) would beaccompanied by the copious production of hard (>10keV) X-ray photons. Theresulting hard X-ray background would redshift and be observed as a present-daysoft X-ray background (SXB). Under the hypothesis that BHs are the mainproducers of reionizing photons in the high-redshift universe, we calculatetheir contribution to the present-day SXB. Our results, when compared to theunresolved component of the SXB in the range 0.5-2 keV, suggest that accretingBHs (be it luminous quasars or their lower-mass ``miniquasar'' counterparts)did not dominate reionization. Distant miniquasars that produce enough X-raysto 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 unresolvedcomponent of the SXB.Comment: 9 emulateapj pages, 2 figures included, submitted to Ap