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

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 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 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.