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 upwards. Current consensus is that at bright magnitudes ($M_{1450}\le -27$) 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$\le z\le$4.2, with magnitudes $i_{psf}\le$18, in an area of 12400$de{g}^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 $\beta =-4.025$ and a space density at $<M_{1450}>=-28.75$ of 2.61$\times 10^{-10}Mp{c}^{-3}$ comoving, after taking into account the estimated incompleteness of our observations. Taking into account the results of fainter surveys, AGN brighter than $M_{1450}=-23$ could produce at least half of the ionizing emissivity at z$\sim$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 $Log(\Gamma [s^{-1}])=-12.17_{-0.07}^{+0.13}$, produced by AGN at M${}_{1450}<-18$, i.e. ~100% of the measured ionizing background at z~4.

Accepted for publication in ApJ, 15 pages, 5 figures, 4 tables

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