The Luminosity Function of Bright QSOs at z ∼ 4 and Implications for the Cosmic Ionizing Background

Abstract

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 ( M 1450 ≤ −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 deg 2 . 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 M 1450 = −29.5. We derived a bright-end slope of β = −4.025 and a space density at 〈 M 1450 〉 = −28.75 of 2.61 × 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 M 1450 = −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 , produced by AGNs at M 1450 < −18, that is, ∼100% of the measured ionizing background at z ∼ 4.