2007ApJ...660.1051S

Using the results of recent optical surveys, we conclude that the nondetection of quasars down to faint magnitudes implies a significant flattening of the high-redshift (z∼6) optical active galactic nucleus (AGN) luminosity function for M₁₄₅₀≳-24.7. We find that all the data are consistent with a faint-end slope for the optical AGN luminosity function of β=-2.2 and -2.8, at the 90% and 99% confidence level, respectively, which is flatter than the bright-end slope of β'~-3.2. We also show that X-ray deep surveys have probed even fainter magnitudes than the optical ones, yielding more significant constraints on the shallow faint-end slope of the optical luminosity function. The inclusion of type II AGN candidates, detected in the Chandra Deep Fields, hints toward a higher normalization for the total AGN luminosity function, if these sources lie at 5≲z≲6.5. We then discuss simple theoretical models of AGN formation and evolution in the context of cold dark matter cosmology. The comparison with the total AGN luminosity function favors a redshift-dependent relation between black hole and dark matter halo masses of the type M_ø∝M^α_halo, with 1.3≲α≲1.7, compatible with independent studies from statistical analysis and rotation curve measurements. Finally, we compute the quasar contribution to reionization to be ≲9% at z∼6, up to ∼30% when integrated within 5.5≲z≲6.5, significantly smaller than that from galaxies.