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 M₁₄₅₀ ∼ -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 (M₁₄₅₀ ∼ -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 (M₁₄₅₀ ∼ -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 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 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.