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

By E. Giallongo, A. Grazian, F. Fiore, A. Fontana, L. Pentericci, E. Vanzella, Mark Dickinson, D. Kocevski, M. Castellano, S. Cristiani and 7 more. 2015

249 citations recorded by the source corpus.

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 \textgreater 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 \textless= 27) using reliable photometric redshifts. At z \textgreater 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 similar to 1.5 x 10(-17) erg cm(-2) s(-1) (0.5-2 keV), corresponding to a probability of spurious detections of 2 x 10(-4) in the deep X-ray 4 Ms Chandra image. Results. We have found 22 AGN candidates at z \textgreater 4 and we have derived the first estimate of the UV luminosity function in the redshift interval 4 \textless z \textless 6.5 and absolute magnitude interval -22.5 less than or similar to M-1450 less than or similar to -18.5 typical of local Seyfert galaxies. The faint end of the derived luminosity function is about two to four magnitudes fainter at z similar to 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-alpha forest of high redshift QSO spectra highly ionized, providing an important contribution to the cosmic reionization.

Subfields: Astronomy and Astrophysics, Instrumentation

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Citations in the corpus

Blossoms from black hole seeds: properties and early growth regulated by supernova feedback (2016, background): “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. c© 0000 RAS, MNRAS 000, 000–000”
REIONIZATION AFTER PLANCK: THE DERIVED GROWTH OF THE COSMIC IONIZING EMISSIVITY NOW MATCHES THE GROWTH OF THE GALAXY UV LUMINOSITY DENSITY (2015, background): “However, even with such steps, quasars appear quite unlikely to contribute meaningfully to the reionization of the universe at z > 6 (but see Giallongo et al. 2015).”
Rapid Reionization by the Oligarchs: The Case for Massive, UV-bright, Star-forming Galaxies with High Escape Fractions (2019, methodology): “This is supported by latest determinations of AGN luminosity functions that limit their contribution to <5% at z=6 (e.g., Kulkarni et al. 2019, though see Giallongo et al. 2015; Boutsia et al. 2018).”
The Evolution of the Intergalactic Medium (2015, background): “However, Giallongo et al. (2015) recently claimed to find a sufficient abundance of quasars at z ∼ 6 for them to be able to reionize the Universe, partially driving a resurgence of quasar-reionization models (Chardin et al. 2015; Madau & Haardt 2015).”
SILVERRUSH. IV. Lyα luminosity functions at z = 5.7 and 6.6 studied with ∼1300 Lyα emitters on the 14–21 deg2 sky (2017, result): “…2012), some previous studies suggest the existence of (faint) AGNs at z ∼ 6− 7 (e.g., Willott et al. 2010; Mortlock et al. 2011; Kashikawa et al. 2015; Giallongo et al. 2015; Jiang et al. 2016; Bowler et al. 2017b; Parsa et al. 2017 ), which may systematically enhance the bright end of our z=5.7…”
Observational evidence for intermediate-mass black holes (2017, unknown): “The use of NIR photometry has allowed us to reach fainter X-ray sources than direct X-ray surveys: using an H -band luminosity selection, Giallongo et al. (2015) identified three faint AGN candidates at z > 6 with X-ray luminosities in the 2-10 keV band above 1043 erg s−1.”
The Rise of Faint, Red Active Galactic Nuclei at z > 4: A Sample of Little Red Dots in the JWST Extragalactic Legacy Fields (2025, unknown): “…abundance of UV-bright quasars implies that such objects are not contributors at z > 6 ( e.g., P. F. Hop-kins et al. 2007 ) , the uncertain abundance of UV-faint AGN leaves the door open for a modest contribution by such sources ( e.g., E. Giallongo et al. 2015; S. L. Finkelstein et al. 2019 ) .”
COSMIC REIONIZATION AFTER PLANCK: COULD QUASARS DO IT ALL? (2015, methodology): “A novel detection criterion is adopted in Giallongo et al. (2015), whereby high-redshift galaxies are first selected in the NIR H band using photometric redshifts, and become AGN candidates if detected in X-rays by Chandra.”
Radiation Backgrounds at Cosmic Dawn: X-Rays from Compact Binaries (2016, background): “…this is indeed the case, it is fair to point out that the demography of faint AGNs at z ∼> 4 is still under debate (see, e.g., Masters et al. 2012; Giallongo et al. 2015), and that the idea of a binary-dominated X-ray background at very early times must be regarded as a working hypothesis at…”
Consistent modelling of the meta-galactic UV background and the thermal/ionization history of the intergalactic medium (2018, methodology): “6.5 by Giallongo et al. (2015) has sparked new interest in scenarios in which AGN contribute significantly to the ionizing photon budget during hydrogen reionization or at least at its tail end.”
An extreme [O III] emitter at z = 3.2: a low metallicity Lyman continuum source (2015, background): “One of the most pressing, unanswered questions in cosmology and galaxy evolution is which are the sources responsible for the reionization of the intergalactic medium (IGM; Robertson et al. 2015) and capable of keeping it ionized afterwards (Fontanot et al. 2014; Giallongo et al. 2015).”
Black hole evolution – I. Supernova-regulated black hole growth (2015, background): “…sensitive to 105–106 M BHs accreting at 1–10 per cent Eddington (Willott 2011; Cowie, Barger & Hasinger 2012; Fiore et al. 2012; Treister et al. 2013; Giallongo et al. 2015; Weigel et al. 2015), but very few, if any are found, implying strong limits on the early growth of BHs in relatively small…”
THE CHANDRA COSMOS LEGACY SURVEY: OVERVIEW AND POINT SOURCE CATALOG (2016, background): “Instead, for the faint and the high-z AGN population, that could be responsible for the re-ionization of the Universe (see Giallongo et al. 2015), Chandra (Weisskopf et al. 2002) is the preferred instrument.”
Evolution of the AGN UV luminosity function from redshift 7.5 (2018, methodology): “• In Model 3, we again exclude samples from Giallongo et al. (2015), Mortlock et al. (2011), Venemans et al. (2015), and Bañados et al. (2018) from the analysis (samples 17–20 from Table 1).”
THE FINAL SDSS HIGH-REDSHIFT QUASAR SAMPLE OF 52 QUASARS AT z > 5.7 (2016, methodology): “Giallongo et al. (2015) found a sample of very faint AGN candidates with < -”
Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs). V. Quasar Luminosity Function and Contribution to Cosmic Reionization at z = 6 (2018, background): “On the other hand, Giallongo et al. (2015) reported Chandra X-ray detection of five very faint active galactic nuclei (AGNs) at z ∼ 6, with −19 ≤ M1450 ≤ −21 mag, over 170 arcmin2 of the Great Observatories Origins Deep Survey (GOODS; Giavalisco et al. 2004) field.”
Great Optically Luminous Dropout Research Using Subaru HSC (GOLDRUSH). I. UV Luminosity Functions at $z \sim 4-7$ Derived with the Half-Million Dropouts on the 100 deg$^2$ Sky (2017, result): “Note that we also show the results of the faint end of the AGN UV LFs (Giallongo et al. 2015; Parsa et al. 2017) in the magnitude range of MUV >∼ −22 mag in Figures 7 and 8.”