Faint AGNs atz>4 in the CANDELS GOODS-S field: looking for contributors to the reionization of the Universe

Abstract

Context. Establishing the number of faint active galactic nuclei (AGNs) at z = 4−6 is crucial to understanding their cosmological importance as main contributors to the reionization of the Universe. Aims. In order to derive the AGN contribution to the cosmological ionizing emissivity we have selected faint AGN candidates at z > 4 in the CANDELS GOODS-South field, which is one of the deepest fields with extensive multiwavelength coverage from Chandra, HST, Spitzer, and various ground-based telescopes. Methods. We have adopted a relatively novel criterion. As a first step, high redshift galaxies are selected in the NIR H band down to very faint levels (H ≤ 27) using reliable photometric redshifts. At z > 4 this corresponds to a selection criterion based on the galaxy rest-frame UV flux. AGN candidates are then picked up from this parent sample if they show X-ray fluxes above a threshold of F X ∼ 1.5 × 10 −17 erg cm −2 s −1 (0.5−2 keV), corresponding to a probability of spurious detections of 2 × 10 −4 in the deep X-ray 4 Ms Chandra image. Results. We have found 22 AGN candidates at z > 4 and we have derived the first estimate of the UV luminosity function in the redshift interval 4 < z < 6.5 and absolute magnitude interval −22.5 M 1450 −18.5 typical of local Seyfert galaxies. The faint end of the derived luminosity function is about two to four magnitudes fainter at z ∼ 4−6 than that derived from previous UV surveys. We estimated ionizing emissivities and hydrogen photoionization rates in the same redshift interval under reasonable assumptions and after discussion of possible caveats, the most important being the large uncertainties involved in the estimate of photometric redshift for sources with featureless, almost power-law SEDs and/or low average escape fraction of ionizing photons from the AGN host galaxies. Both effects could, in principle, significantly reduce the estimated average volume densities and/or ionizing emissivities, especially at the highest redshifts. Conclusions. At z = 4−6.5 we argue that, under reasonable evaluations of possible biases, the probed AGN population can produce photoionization rates consistent with that required to keep the intergalactic medium observed in the Lyman-α forest of high redshift QSO spectra highly ionized, providing an important contribution to the cosmic reionization.

Dark-ages reionization and galaxy formation simulation – X. The small contribution of quasars to reionization

“…We see that the model shows good agreement with observations across a large redshift range (z ∼ 6 − 0.6). At high redshift the model is consistent with the samples of bright quasars (Willott et al 2010;Shen & Kelly 2012;McGreer et al 2013;Jiang et al 2016) while it predicts a lower number density of faint quasars compared to the Giallongo et al (2015, hereafter the G15) sample. On the other hand, the model produces a significant number of faint quasars.…”

“…With a high escape fraction (fesc,q ∼ 1, , quasars (AGN) are potential contributors to reionization (Volonteri & Gnedin 2009;Fontanot et al 2014;Madau & Haardt 2015;Mitra et al 2015), despite their relatively low number. Recently, Giallongo et al (2015) identified faint AGN candidates in the CANDELS GOODS-South field and suggested that there is a high number density of faint AGN at z = 4 − 6. These faint quasars provide a new source of reionization (Madau & Haardt 2015).…”

Blossoms from black hole seeds: properties and early growth regulated by supernova feedback

“…10), the BH luminosity function of this model is the closest to the bolometric and X-ray luminosity functions derived respectively by Hopkins, Richards & Hernquist (2007), and Buchner et al (2015) (Fig. 12), and finally the model provides a number of AGN in better agreement with the number of AGN candidates detected in the CDF-S survey (Giallongo et al 2015) at high redshift. Figure 13.…”

Cosmic reionization after Planck

“…The contribution of quasars are incorporated here based on their observed luminosity function at z < 6 (Hopkins et al 2007) and they have insignificant effects on IGM at higher redshifts (but also see Giallongo et al 2015;Madau & Haardt 2015). Furthermore, the model calculates the suppression of star formation in low-mass haloes (radiative feedback) through a Jeans mass prescription, which is computed self-consistently from the evolution of the thermal properties of IGM through the minimum circular velocity of haloes that are able to cool (Choudhury & Ferrara 2005; also comparable to the simulations by Okamoto et al 2008;Sobacchi & Mesinger 2013).…”