High-redshift active galactic nuclei and H I reionisation: limits from the unresolved X-ray background

¹ DiSAT, Università dell’Insubria, via Valleggio 11, 22100 Como, Italy
e-mail: haardt@mib.infn.it
² INFN, Sezione di Milano-Bicocca, Piazza delle Scienze 3, 20123 Milano, Italy
³ INAF, IASF Milano, via E. Bassini 15, 20133 Milano, Italy

Received: 8 January 2015
Accepted: 10 February 2015

Abstract

The rapidly declining population of bright quasars at z ≳ 3 appears to make an increasingly smaller contribution to the ionising background at the H I Lyman limit. It is thererfore generally thought that massive stars in (pre-)Galactic systems may provide the additional ionising flux needed to complete H I reionisation by z ≳ 6. A galaxy-dominated background, however, may require that the escape fraction of Lyman continuum radiation from high-redshift galaxies is as high as 10%, which is somewhat at odds with (admittedly scarce) observational constraints. High escape fractions from dwarf galaxies have been advocated, or, alternatively, a so-far undetected (or barely detected) population of unobscured, high-redshift faint AGNs. Here we examine the latter hypothesis and show that such sources, to be consistent with the measured level of the unresolved X-ray background at z = 0, can provide a fraction of the H II filling factor no larger than 13% by z ≃ 6. The fraction rises to ≲27% in the somewhat extreme case of a constant comoving redshift evolution of the AGN emissivity. This still calls for a mean escape fraction of ionising photons from high-z galaxies of ≳10%.