On the AGN Nature of Two UV-bright Sources at z spec  ∼ 5.5 in the CANDELS Fields: An Update on the AGN Space Density at M 1450  ∼ −22.5

Abstract

It is widespread opinion that hydrogen reionization is mainly driven by primeval star-forming galaxies, with high- z active galactic nuclei (AGNs) having a minor role. 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 ( M 1450 ∼ −22.5) AGNs at z spec  ∼ 5.5 in order to improve the estimate of the photoionization rate contribution from accreting supermassive 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 ( M 1450 ∼ −22.5) AGNs at z spec  ∼ 5.5. We derive an AGN space density of Φ = 1.29 × 10 −6 cMpc −3 at z  ∼ 5.5 and M 1450  ∼ −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 photoionization 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 James Webb Space Telescope and giant ground-based telescopes observations will improve the study of high- z AGNs and their contribution to the reionization of the universe.