High redshift AGNs and HI reionisation: limits from the unresolved X-ray background

TL;DR

This paper investigates whether faint high-redshift AGNs can significantly contribute to HI reionisation and concludes they are unlikely to be the main source, emphasizing the need for high escape fractions from galaxies.

Contribution

It challenges the hypothesis that faint AGNs dominate HI reionisation, providing constraints based on the unresolved X-ray background.

Findings

Faint high-z AGNs contribute at most 13% to HII filling factor by z=6.

Contribution increases to less than 27% under extreme assumptions.

High escape fractions (>10%) from galaxies are still necessary for reionisation.

Abstract

The rapidly declining population of bright quasars at z~3 appears to make an increasingly small contribution to the ionising background at the HI Lyman limit. It is then generally though that massive stars in (pre-)galactic systems may provide the additional ionising flux needed to complete HI 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%, a value 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 question 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 HII filling factor not 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 >10%.