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

Journal
Date Issued
2020
Author(s)
•
•
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Boutsia, K.
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Civano, F.
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•
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Dickinson, M.
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•
•
Romano, M.
DOI
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 (M1450 ∼ -22.5) AGNs at zspec ∼ 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 (M1450 ∼ -22.5) AGNs at zspec ∼ 5.5. We derive an AGN space density of Φ = 1.29 × 10-6 cMpc-3 at z ∼ 5.5 and M1450 ∼ -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.
Volume
897
Issue
1
Start page
94
Uri
Url
Issn Identifier
0004-637X
Ads BibCode
2020ApJ...897...94G
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open.access
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