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

TL;DR

This study updates the space density of faint AGNs at redshift 5.5, suggesting a higher abundance than previously thought, and supports their significant role in cosmic reionization.

Contribution

It provides new observational evidence for a higher space density of faint AGNs at z~5.5, challenging previous underestimations based on color selection methods.

Findings

Detected two AGNs at z~5.5 with M1450~-22.5.

Derived a lower-limit AGN space density of 1.29x10^-6 cMpc^-3.

Supports a significant contribution of AGNs to reionization at high redshift.

Abstract

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.