On the faint end of the high-redshift active galactic nucleus luminosity function

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 similar to 6) optical active galactic nucleus (AGN) luminosity function for M-1450 greater than or similar to - 24.7. We find that all the data are consistent with a faint- end slope for the optical AGN luminosity function of beta = - 2.2 and -2.8, at the 90% and 99% confidence level, respectively, which is flatter than the bright- end slope of beta' 0 similar to - 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 less than or similar to z less than or similar to 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-circle proportional to M-halo(alpha), with 1.3 less than or similar to alpha less than or similar to 1.7, compatible with independent studies from statistical analysis and rotation curve measurements. Finally, we compute the quasar contribution to reionization to be P9% at z similar to 6, up to similar to 30% when integrated within 5.5 less than or similar to z less than or similar to 6.5, significantly smaller than that from galaxies.