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<SUB>1450</SUB> ≤ -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<SUB>psf</SUB> ≤ 18, in an area of 12400 deg<SUP>2</SUP>. 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<SUB>1450</SUB> = -29.5. We derived a bright-end slope of β = -4.025 and a space density at <M<SUB>1450</SUB>> = -28.75 of 2.61 × 10<SUP>-10</SUP> Mpc<SUP>-3</SUP> 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<SUB>1450</SUB> = -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 ${\rm{Log}}({\rm{\Gamma }}\left[{{\rm{s}}}^{-1}\right])=-{12.17}_{-0.07}^{+0.13}$ , produced by AGNs at M<SUB>1450</SUB> < -18, that is, ∼100% of the measured ionizing background at z ∼ 4.