Title: No evidence for a significant AGN contribution to cosmic hydrogen reionization,
Abstract: We reinvestigate a claimed sample of 22 X-ray detected active galactic nuclei
(AGN) at redshifts z > 4, which has reignited the debate as to whether young
galaxies or AGN reionized the Universe. These sources lie within the
GOODS-S/CANDELS field, and we examine both the robustness of the claimed X-ray
detections (within the Chandra 4Ms imaging) and perform an independent analysis
of the photometric redshifts of the optical/infrared counterparts. We confirm
the reality of only 15 of the 22 reported X-ray detections, and moreover find
that only 12 of the 22 optical/infrared counterpart galaxies actually lie
robustly at z > 4. Combining these results we find convincing evidence for only
7 X-ray AGN at z > 4 in the GOODS-S field, of which only one lies at z > 5. We
recalculate the evolving far-UV (1500 Angstrom) luminosity density produced by
AGN at high redshift, and find that it declines rapidly from z = 4 to z = 6, in
agreement with several other recent studies of the evolving AGN luminosity
function. The associated rapid decline in inferred hydrogen-ionizing emissivity
contributed by AGN falls an order-of-magnitude short of the level required to
maintain hydrogen ionization at z ~ 6. We conclude that all available evidence
continues to favour a scenario in which young galaxies reionized the Universe,
with AGN making, at most, a very minor contribution to cosmic hydrogen
reionization.