A Survey of CLCITALz/ITAL/CLC > 5.8 Quasars in the Sloan Digital Sky Survey. I. Discovery of Three New Quasars and the Spatial Density of Luminous Quasars at CLCITALz/ITAL/CLC ∼ 6

We present the results from a survey of i-dropout objects selected from ~1550 deg² of multicolor imaging data from the Sloan Digital Sky Survey, to search for luminous quasars at z>5. 8. Objects with i*-z*>2. 2 and z* 0. 90. The ARC 3. 5m spectrum of the z=6. 28 quasar shows that over a range of 300 A immediately blueward of the Ly alpha emission, the average transmitted flux is only 0. 003 +/-0. 020 times that of the continuum level, consistent with zero flux, and suggesting a tentative detection of the complete Gunn-Peterson trough. The existence of strong metal lines suggests early chemical enrichment in the quasar enviornment. The three new objects, together with the previously published z=5. 8 quasar form a complete color-selected flux-limited sample at z>5. 8. We estimate that at z=6, the comoving density of luminous quasars at M₁450 < -26. 89 (h=0. 5, Omega=1) is 1. 1x10^-9 Mpc^-3. This is a factor of ~2 lower than that at z~5, and is consistent with an extrapolation of the observed quasar evolution at low-z. We discuss the contribution of quasars to the ionizing background at z~6. The luminous quasars discussed in the paper have central black hole masses of several times 10⁹ Mₛun by the Eddington argument. Their observed space density provides a sensitive test of models of quasar and galaxy formation at high redshift. (Abridged)