2005ApJ...633..630W

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 i' and z' filters to a 10 σ limit of z'<23.35. Near-infrared photometry of 24 candidate 5.7<z<6.4 quasars confirms them all to be low-mass 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^*₁₄₅₀and faint-end slope α, we determine that the bright-end slope β>-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.