The Luminosity Function of bright QSOs at z~4 and implications for the cosmic ionizing background

Abstract

Based on results by recent surveys, the number of bright quasars at redshifts z>3 is being constantly revised upwards. Current consensus is that at bright magnitudes (M 1450 ≤ −27) the number densities of such sources could have been underestimated by a factor of 30-40%. In the framework of the QUBRICS survey, we identified 58 bright QSOs at 3.6≤ z ≤4.2, with magnitudes i psf ≤18, in an area of 12400deg 2 . The uniqueness of our survey is underlined by the fact that it allows us, for the first time, to extend the sampled absolute magnitude range up to M 1450 = −29.5. We derived a bright-end slope of β = −4.025 and a space density at < M 1450 >= −28.75 of 2.61×10 −10 M pc −3 comoving, after taking into account the estimated incompleteness of our observations. Taking into account the results of fainter surveys, AGN brighter than M 1450 = −23 could produce at least half of the ionizing emissivity at z∼4. Considering a mean escape fraction of 0.7 for the QSO and AGN population, combined with a mean free path of 41.3 proper Mpc at z = 3.9, we derive a photoionization rate of Log(Γ[s −1 ]) = −12.17 +0.13 −0.07 , produced by AGN at M 1450 < −18, i.e. ∼100% of the measured ionizing background at z∼4.

Ultraluminous high-redshift quasars from SkyMapper – II. New quasars and the bright end of the luminosity function

“…We note that at a slightly lower redshift of ∼ 3.9, Boutsia et al (2021) found a similar bright-end slope of ≈ −4, which we confirm perfectly in our cumulative count method applied in Fig. 7, finding = −4.05 ± 0.03.…”

“…However, most of the past works could not draw on complete samples of QSOs at the extreme luminosities of 145 < −28, and hence we now compare the newest bright-end measurements of the QSO LF spanning the redshift range from ∼ 4 to ∼ 6 to reassess its evolution. We complement our work with the LF by Boutsia et al (2021) on the lower-z side and with the LF by Jiang et al (2009) on the higher-z end. Here, we need to visualise the actual best-fit parametrisations, because, due to the range of different bright-end slopes, it would be insufficient to consider only * and Φ * parameters.…”

“…However, we can also look at hemispheric comparisons at other redshifts, and their behaviour should show a scatter of larger as well as smaller densities, because the cosmic volumes of different redshift bins sample uncorrelated parts of the large-scale structure and the volume in each redshift bin is huge, on the order of 50-100 Gpc 3 . For this purpose we compare luminosity functions determined by different authors: our work as well as the recent LF by Boutsia et al (2021) are derived in the Southern hemisphere and, overall, span redshifts of 3.6 < < 5.5. Most literature samples from the past were constrained by the SDSS footprint in the North, such as the = 6 LF by Jiang et al (2009) and the LF by Y16, centred on = 5.05.…”

Ionizing radiation from AGNs at z>3.3 with the Subaru Hyper Suprime-Cam Survey and the CFHT Large Area U-band Deep Survey (CLAUDS)

“…Giallongo et al (2015) argued that, based on their selection of faint AGN candidates in the GOODS-S field with photometric redshift and X-ray detection, the number density of faint AGNs at 4 < z < 6.5 is much higher than those estimated from the existing luminosity function reported in literature, and suggested that LyC emissivity by AGNs could provide ionizing photons sufficient to keep the IGM ionized. For the redshift range 4 < z < 4.5 their estimate of LyC is 1.15 × 10 25 erg/s/Hz/Mpc 3 , which exceeds the nominal value of LyC from all sources at z ∼ 4 reported by Becker & Bolton (2013) (9.62 × 10 24 erg/s/Hz/Mpc 3 ; see also Boutsia et al (2021) for the updated UVLF and estimates on the contributions to the LyC emissivity by faint AGNs). We should note that recent results of quasar UVLF at z ∼ 4 and higher based on HSC SSP (Akiyama et al 2018;Matsuoka et al 2018) suggest fewer number density of faint AGNs.…”

“…However, Giallongo et al (2015) argued that the steep faint-end slope of the AGN UVLF they found for X-ray selected AGNs at 4 < z < 6.5 would imply that AGN could provide enough photons to keep the Universe ionized. This view is supported by Boutsia et al (2018) who argued that, based on spectroscopy of faint AGNs in the COSMOS field, the number density of faint AGNs could be higher than found by earlier studies, and that, consequently, AGNs could make a substantial contribution to the ionizing photon budget (see also Giallongo et al (2019), Grazian et al (2020) and Boutsia et al (2021) for further reports on high AGN space density at z 4). Clearly, an accurate determination of the AGN UVLF is critically important to give us a definitive evaluation of the AGN contribution to the ionizing photon budget.…”