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The Space Density of Ultra-luminous QSOs at the End of Reionization Epoch by the QUBRICS Survey and the AGN Contribution to the Hydrogen Ionizing Background
Ist Teil von
The Astrophysical journal, 2022-01, Vol.924 (2), p.62
Ort / Verlag
Philadelphia: The American Astronomical Society
Erscheinungsjahr
2022
Quelle
EZB Electronic Journals Library
Beschreibungen/Notizen
Abstract
Motivated by evidences favoring a rapid and late hydrogen reionization process completing at
z
∼ 5.2–5.5 and mainly driven by rare and luminous sources, we have reassessed the estimate of the space density of ultra-luminous QSOs at
z
∼ 5 in the framework of the QUBRICS survey. A ∼ 90% complete sample of 14 spectroscopically confirmed QSOs at
M
1450
≤ −28.3 and 4.5 ≤
z
≤ 5.0 has been derived in an area of 12,400 deg
2
, thanks to multiwavelength selection and Gaia astrometry. The space density of
z
∼ 5 QSOs within −29.3 ≤
M
1450
≤ −28.3 is three times higher than previous determinations. Our results suggest a steep bright-end slope for the QSO luminosity function at
z
∼ 5 and a mild redshift evolution of the space density of ultrabright QSOs (
M
1450
∼ −28.5) at 3 <
z
< 5.5, in agreement with the redshift evolution of the much fainter active galactic nucleus (AGN) population at
M
1450
∼ −23. These findings are consistent with a pure density evolution for the AGN population at
z
> 3. Adopting our
z
∼ 4 QSO luminosity function and applying a mild density evolution in redshift, a photoionization rate of
Γ
HI
=
0.46
−
0.09
+
0.17
×
10
−
12
s
−
1
has been obtained at
z
= 4.75, assuming an escape fraction of ∼70% and a steep faint-end slope of the AGN luminosity function. The derived photoionization rate is ∼50–100% of the ionizing background measured at the end of the reionization epoch, suggesting that AGNs could play an important role in the cosmological reionization process.