First Results from the Canada-France High-z Quasar Survey: Constraints on the z = 6 Quasar Luminosity Function and the Quasar Contribution to Reionization

We present preliminary results of a new quasar survey being undertaken with multicolor optical imaging from the Canada-France-Hawaii Telescope. The current data consist of 3.83 deg2 of imaging in the Iʹ and zʹ filters to a 10 (sigma)limit of zʹ < 23.35. Nearinfrared photometry of 24 candidate 5.7 < z < 6.4 quasars confirms them all to be lowmass stars, including two T dwarfs and four or five L dwarfs. Photometric estimates of the spectral type of the two T dwarfs are T3 and T6. We use the lack of high-redshift quasars in this survey volume to constrain the z = 6 quasar luminosity function. For reasonable values of the break absolute magnitude M... and faint-end slope (alpha), we determine that the bright-end slope (beta)> -3.2 at 95% confidence. We find that the comoving space density of quasars brighter than M1450 = -23.5 declines by a factor of >25 from z = 2 to 6, mirroring the decline observed for high-luminosity quasars. We consider the contribution of the quasar population to the ionizing photon density at z = 6 and the implications for reionization. We show that the current constraints on the quasar population give an ionizing photon density <<30% that of the star-forming galaxy population. We conclude that active galactic nuclei make a negligible contribution to the reionization of hydrogen at z ~6.