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http://hdl.handle.net/20.500.12386/31879
Title: | On the AGN Nature of Two UV-bright Sources at zspec ∼ 5.5 in the CANDELS Fields: An Update on the AGN Space Density at M1450 ∼ −22.5 | Authors: | GRAZIAN, Andrea GIALLONGO, Emanuele FIORE, Fabrizio Boutsia, K. Civano, F. CRISTIANI, Stefano CUPANI, Guido Dickinson, M. FONTANOT, Fabio MENCI, Nicola Romano, M. |
Issue Date: | 2020 | Journal: | THE ASTROPHYSICAL JOURNAL | Number: | 897 | Issue: | 1 | First Page: | 94 | Abstract: | It is widespread opinion that hydrogen reionization is mainly driven by primeval star-forming galaxies, with high-z active galactic nuclei (AGNs) having a minor role. 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<SUB>1450</SUB> ∼ -22.5) AGNs at z<SUB>spec</SUB> ∼ 5.5 in order to improve the estimate of the photoionization rate contribution from accreting supermassive 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<SUB>1450</SUB> ∼ -22.5) AGNs at z<SUB>spec</SUB> ∼ 5.5. We derive an AGN space density of Φ = 1.29 × 10<SUP>-6</SUP> cMpc<SUP>-3</SUP> at z ∼ 5.5 and M<SUB>1450</SUB> ∼ -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 photoionization 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 James Webb Space Telescope and giant ground-based telescopes observations will improve the study of high-z AGNs and their contribution to the reionization of the universe. | URI: | http://hdl.handle.net/20.500.12386/31879 | URL: | https://iopscience.iop.org/article/10.3847/1538-4357/ab99a3 https://api.elsevier.com/content/abstract/scopus_id/85088039042 |
ISSN: | 0004-637X | DOI: | 10.3847/1538-4357/ab99a3 | Bibcode ADS: | 2020ApJ...897...94G | Fulltext: | open |
Appears in Collections: | 1.01 Articoli in rivista |
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2006.02451.pdf | preprint | 855.69 kB | Adobe PDF | View/Open |
Grazian20.pdf | [Administrators only] | 1.02 MB | Adobe PDF |
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