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 M₁₄₅₀ <= -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 <= M₁₄₅₀ <= -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 (M₁₄₅₀ ∼ -28.5) at 3 < z < 5.5, in agreement with the redshift evolution of the much fainter active galactic nucleus (AGN) population at M₁₄₅₀ ∼ -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^–12s^–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.