The contribution of faint AGNs to the ionizing background at z ~ 4

Abstract

Context. Finding the sources responsible for the hydrogen reionization is one of the most pressing issues in observational cosmology. Bright QSOs are known to ionize their surrounding neighborhood, but they are too few to ensure the required HI ionizing background. A significant contribution by faint AGNs, however, could solve the problem, as recently advocated on the basis of a relatively large space density of faint active nuclei at z > 4. Aims. This work is part of a long term project aimed at measuring the Lyman Continuum escape fraction for a large sample of AGNs at z ∼ 4 down to an absolute magnitude of M 1450 ∼ −23. We have carried out an exploratory spectroscopic program to measure the HI ionizing emission of 16 faint AGNs spanning a broad U − I color interval, with I ∼ 21 − 23 and 3.6 < z < 4.2. These AGNs are three magnitudes fainter than the typical SDSS QSOs (M 1450 −26) which are known to ionize their surrounding IGM at z 4. Methods. We acquired deep spectra of these faint AGNs with spectrographs available at the VLT, LBT, and Magellan telescopes, i.e. FORS2, MODS1-2, and LDSS3, respectively. The emission in the Lyman Continuum region, i.e. close to 900 Å rest frame, has been detected with S/N ratio of ∼ 10 − 120 for all the 16 AGNs. The flux ratio between the 900 Å rest frame region and 930 Å provides a robust estimate of the escape fraction of HI ionizing photons. Results. We have found that the Lyman Continuum escape fraction is between 44 and 100% for all the observed faint AGNs, with a mean value of 74% at 3.6 < z < 4.2 and −25.1 M 1450 −23.3, in agreement with the value found in the literature for much brighter QSOs (M 1450 −26) at the same redshifts. The Lyman Continuum escape fraction of our faint AGNs does not show any dependence on the absolute luminosities or on the observed U − I colors of the objects. Assuming that the Lyman Continuum escape fraction remains close to ∼ 75% down to M 1450 ∼ −18, we find that the AGN population can provide between 16 and 73% (depending on the adopted luminosity function) of the whole ionizing UV background at z ∼ 4, measured through the Lyman forest. This contribution increases to 25-100% if other determinations of the ionizing UV background are adopted from the recent literature. Conclusions. Extrapolating these results to z ∼ 5 − 7, there are possible indications that bright QSOs and faint AGNs can provide a significant contribution to the Reionization of the Universe, if their space density is high at M 1450 ∼ −23.

Lyman continuum escape fraction and mean free path of hydrogen ionizing photons for brightz∼ 4 QSOs from SDSS DR14

“…A long-lasting debate is in progress on the sources responsible for the EoR. While the majority of the astrophysical community are favoring star-forming galaxies (SFGs) as the main drivers of HI reionization (Dayal & Ferrara 2018), alternative explanations based on Active Galactic Nuclei (AGNs) have been proposed (Giallongo et al 2012(Giallongo et al , 2015Madau & Haardt 2015;Grazian et al 2018;Boutsia et al 2018). It is widely accepted that QSOs and AGNs dominate the HI ionizing background at z < 3 (Haardt & Madau 2012;Fontanot et al 2012;Cristiani et al 2016;Faucher-Giguère 2019).…”

AGN jets versus accretion as reionization sources

“…In order to do this, we took the LF of Matsuoka et al (Matsuoka et al 2018, corrected for absorption), starforming galaxies (Robertson et al 2015), and AGN jets (constant obscuration, CO, and differential obscuration, DO) for = 1 (maximum possible contributions). The blue lines are the contributions of AGN disks assuming an average f esc = 0.74 as in Grazian et al (2018) (2018) and corrected it in three different ways to account for the presence of obscured sources. We considered an obscuration factor constant at all AGN luminosities (CO), fainter sources being more obscured than brighter ones (DO), and an LF with no turnover (NT).…”

“…Wagner et al 2013;Menci et al 2019) even in obscured sources. Grazian et al (2018) analyzed a sample of 16 AGN at z ∼4, including both obscured and unobscured nuclei, and estimated an average escape fraction of ionizing radiation of f esc = 0.74.…”

“…However, given the observations of Grazian et al (2018) at z ∼4 and the lack of complete, large X-ray samples at higher redshift, the possibility of large escape fractions for AGN disk photons cannot be completely ruled out either.…”

“…In this way, the plots in Fig. 3 show a range of possible values of the AGN disk contribution to reionization, from the small correction to the Matsuoka et al (2018) LF to the large f esc of Grazian et al (2018).…”