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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

Grazian, Andrea et al., 2022, The Astrophysical Journal, 924, 62 | View on ADS (2022ApJ...924...62G)


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 deg2, 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 ${{\rm{\Gamma }}}_{\mathrm{HI}}={0.46}_{-0.09}^{+0.17}\times {10}^{-12}\,{{\rm{s}}}^{-1}$ 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.

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