Quasar UV luminosity function evolution up to z=8

Abstract

We study the redshift evolution of the quasar UV Luminosity Function (LF) for 0.5 < z < 6.5, by collecting the most up to date observational data and, in particular, the recently discovered population of faint AGNs. We fit the QSO LF using either a double power-law or a Schechter function, finding that both forms provide good fits to the data. We derive empirical relations for the LF parameters as a function of redshift and, based on these results, predict the quasar UV LF at z=8. From the inferred LF evolution, we compute the redshift evolution of the QSO/AGN comoving ionizing emissivity and hydrogen photoionization rate. If faint AGNs are included, the contribution of quasars to reionization increases substantially. However, their level of contribution critically depends on the detailed shape of the QSO LF, which can be constrained by efficient searches of high-z quasars. To this aim, we predict the expected (i) number of z>6 quasars detectable by ongoing and future NIR surveys (as EUCLID and WFIRST), and (ii) number counts for a single radio-recombination line observation with SKA-MID (FoV = 0.49 deg^2) as a function of the Hnalpha flux density, at 0<z<8. These surveys (even at z<6) will be fundamental to better constrain the role of quasars as reionization sources.