A Limit from the X‐Ray Background on the Contribution of Quasars to Reionization

Abstract

A population of black holes ( BHs) at high redshifts (z k 6) that contributes significantly to the ionization of the intergalactic medium (IGM) would be accompanied by the copious production of hard (k10 keV ) 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. We find that accreting BHs with a hard spectrum (be it luminous quasars or their lower mass ''miniquasar'' counterparts) could not fully reionize the universe without saturating the unresolved component of the 0.5-2 keV SXB at the !2 level. Distant miniquasars that produce enough X-rays to only partially ionize the IGM to a level of at most x e $ 50% saturate the unresolved SXB by P1 . Improved determinations of the unresolved component of the SXB can provide a powerful constraint on the contribution of accreting BHs to partial or full reionization. Subject headingg s: cosmology: theory -intergalactic medium -quasars: general On-line material: color figure

Galaxy UV-luminosity function and reionization constraints on axion dark matter

“…AGN feedback could possibly loosen some constraints. Such a model, however, is severely contained by the diffuse X-ray background (Dijkstra, Haiman & Loeb 2004).…”

Reionization and Cosmology with 21-cm Fluctuations

“…For ionization fractions 10%, ionization by secondary electrons becomes inefficient (Ricotti & Ostriker 2004) and X-ray ionization is self regulating at the level of ∼ 10 − 20%. Observational limits on the contribution of X-rays to reionization are provided by the unresolved soft X-ray background, which limits the possible level of ionization by X-rays from a population of black holes at high redshift (Dijkstra, Haiman & Loeb 2004). …”

Secondary anisotropies of the CMB

“…The ionising sources cannot be a population of normal galaxies or known quasars. Optical studies of the bright quasar luminosity function (Haiman, Abel & Madau 2001, Wyithe & Loeb 2003, as well the associated X-ray background (Djikstra, Haiman & Loeb 2004) rule out the known quasar population as a reionisation source. However miniquasars with correspondingly softer spectra could evade this constraint.…”

“…Ricotti & Ostriker 2004, Ricotti, Ostriker & Gnedin 2005, Madau et al 2004, Oh 2001, Dijkstra, Haiman & Loeb 2004. In view of the correlation between central black hole mass and spheroid velocity dispersion (Ferrarese & Merritt 2000, Gebhardt et al 2000, miniquasars are as plausible ionisation sources as are Population III stars, whose nucleosynthetic traces have not yet been seen even in the most metal-poor halo stars nor in the high z Lyman alpha forest.…”