On the AGN nature of two UV bright sources at z_spec~5.5 in the CANDELS
  fields: an update of the AGN space density at M1450~-22.5
  It is a widespread opinion that hydrogen reionization is mainly driven by
primeval star-forming galaxies, with a minor role of high-z active galactic
nuclei. Recent observations, however, challenge this notion, indicating a
number of issues related to a galaxy-driven reionization scenario. We provide
here an updated assessment of the space density of relatively faint
(M1450~-22.5) AGNs at zspec~5.5 in order to improve the estimate of the
photo-ionization rate contribution from accreting super massive black holes.
Exploiting deep UV rest-frame ground-based spectra collected at the Very Large
Telescope on the CANDELS/GOODS-South field and deep Chandra X-ray images in the
CANDELS/GOODS-North and EGS areas, we find two relatively bright (M1450~-22.5)
AGNs at zspec~5.5. We derive an AGN space density of Phi=1.29x10^-6 cMpc^-3 at
z~5.5 and M1450~-22.5 by simply dividing their observed number by the
cosmological volume in the range 5.0<z<6.1. Our estimate does not consider
corrections for incompleteness, therefore it represents a lower limit, although
uncertainties due to cosmic variance can still be significant. This value
supports a high space density of AGNs at z>5, in contrast with previous claims
mostly based on standard color selection, possibly affected by significant
incompleteness. Our estimate for the AGN photo-ionization rate at z~5.5 is in
agreement with the observed values at similar redshifts, which are needed to
keep the intergalactic medium highly ionized. Upcoming JWST and giant ground
based telescopes observations will improve the study of high-z AGNs and their
contribution to the reionization of the Universe.
