2021ApJ...912..111B

Based on results by recent surveys, the number of bright quasars at redshifts z > 3 is being constantly revised upward. The current consensus is that at bright magnitudes (M₁₄₅₀ <= -27) the number densities of such sources could have been underestimated by a factor of 30%-40%. In the framework of the QUBRICS survey, we identified 58 bright QSOs at 3.6 <= z <= 4.2, with magnitudes i_psf <= 18, in an area of 12400 deg². The uniqueness of our survey is underlined by the fact that it allows us, for the first time, to extend the sampled absolute magnitude range up to M₁₄₅₀ = -29.5. We derived a bright-end slope of β = -4.025 and a space density at <M₁₄₅₀> = -28.75 of 2.61 x 10^–10 Mpc^–3 comoving, after taking into account the estimated incompleteness of our observations. Taking into account the results of fainter surveys, active galactic nuclei (AGNs) brighter than M₁₄₅₀ = -23 could produce at least half of the ionizing emissivity at z ∼ 4. Considering a mean escape fraction of 0.7 for the QSO and AGN population, combined with a mean free path of 41.3 proper Mpc at z = 3.9, we derive a photoionization rate of Log(Γ[s^–1])=-12.17_–0.07^+0.13, produced by AGNs at M₁₄₅₀ < -18, that is, ∼100% of the measured ionizing background at z ∼ 4.