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

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 (M 1450 ∼ −22.5) AGNs at z spec ∼ 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 (M 1450 ∼ −22.5) AGNs at z spec ∼ 5.5. We derive an AGN space density of Φ = 1.29 × 10 −6 cM pc −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 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.

Accretion History of AGNs. III. Radiative Efficiency and AGN Contribution to Reionization

“…Therefore, we also compute an upper limit to reionization from AGN, by considering the high escape fraction suggested by Grazian et al (2018) for both unabsorbed and Compton-thin AGN (log N H < 24) at all redshifts and luminosity. Figure 4 shows that the A19-based UV photoionization rate for AGN with log N H < 24 (grey lines) agrees well with the Giallongo et al (2019) and Grazian et al (2020) UV LFs (blue and red points, respectively), both where AGN space densities Table 1. Radiative Efficiency (a) and Spin (b) XLF η kin η (c) Spin A19 (d) 0 0.34 +0.15 −0.16 (0.118 +0.024 −0.020 ) 0.998 +0.000 −0.018 (0.783 +0.096 −0.056 ) 0.15 0.30 +0.12 −0.15 (0.100 +0.021 −0.019 ) 0.9995 +0.0 −0.0012 (0.9832 +0.0039 −0.0034 ) U14 0 0.20 +0.18 −0.10 (0.068 +0.022 −0.023 ) 0.997 +0.000 −0.039 (0.28 +0.20 −0.15 ) 0.15 0.178 +0.162 −0.098 (0.059 +0.021 −0.023 ) 0.99961 +0.0 −0.00081 (0.9632 +0.0056 −0.0037 ) (a) Calculated for different XLFs and values of ρ SMBH .…”

“…Their analysis of several AGN optical and UV LFs (Willott et al 2009(Willott et al , 2010McGreer et al 2013) indicate that quasar emissivity is too low to be a significant contributor to reionization, although at the time faint-end of the quasar luminosity function was rather uncertain. Recently, the faint-end has been updated by Giallongo et al (2019) and Grazian et al (2020), so we compare A19 XLF with these new high redshift quasar number densities as well as with the Bouwens et al (2015b) results. Mason et al (2019) derived a non-parametric form of total Ṅion (z) by combining constrains from optical depth, dark gap statistics, Lyα damping wings of quasars and ratio of Lyα emitters over Lyman break galaxies.…”

“…As an initial check on our XLF to UV LF conversion, we compare our derived photoionization rate to the one from Giallongo et al ( 2019), after multiplying each by a factor of 1.2 to account for the contribution of recombination radiation to the ionizing background (D'Aloisio et al 2018;Faucher-Giguère et al 2009). We find that the summed space densities of unabsorbed and Compton-thin objects from A19 agree well with Giallongo et al (2019) and Grazian et al (2020, the latter provides an updated estimate of the former). Therefore, we also compute an upper limit to reionization from AGN, by considering the high escape fraction suggested by Grazian et al (2018) for both unabsorbed and Compton-thin AGN (log N H < 24) at all redshifts and luminosity.…”

“…In this section we describe our calculation of the AGN contribution to reionization at UV and X-ray wavelengths. Then, after converting the A19 XLF into a UV luminosity function, we compare to the the Giallongo et al (2019) and Grazian et al (2020) UV luminosity function for AGN in the redshift range 4 < z < 6.1. We also compare the total ionizing photon contribution from AGN, derived from the A19 model, with the galaxy contribution and overall reionization budget.…”

The Infrared Medium-deep Survey. VIII. Quasar Luminosity Function at $z\sim5$

“…vealed a large number of faint quasars at high redshifts, supporting a significant contribution of quasars to the IGM ionizing background (Glikman et al 2011;Boutsia et al 2018;Grazian et al 2020). In particular, for Xray-selected quasars with M 1450 ≥ −22 mag, Giallongo et al (2019;hereafter G19) have suggested a substantial contribution of quasars ( 50%, depending on z) to the IGM ionizing background (see also Giallongo et al 2015).…”