A survey of z > 5.7 quasars in the sloan digital sky survey. III. Discovery of five additional quasars

We present the discovery of five new quasars at z > 5.7, selected from the multicolor imaging data of the Sloan Digital Sky Survey (SDSS). Three of them, at redshifts 5.93, 6.07, and 6.22, were selected from ∼1700 deg ² of new SDSS Main Survey imaging in the northern Galactic cap. An additional quasar, at redshift 5.85, was discovered by co-adding the data obtained in the Fall Equatorial Stripe in the SDSS Southern Survey Region. The fifth object, at redshift 5.80, is selected from a nonstandard SDSS scan in the southern Galactic cap outside the Main Survey area. The spectrum of SDSS J162331.81+311200.5 (z = 6.22) shows a complete Gunn-Peterson trough at z _abs > 5.95, similar to the troughs detected in the other three z ≳ 6.2 quasars known. We present a composite spectrum of the z > 5.7 quasars discovered in the SDSS to date. The average emission-line and continuum properties of z ∼ 6 quasars exhibit no significant evolution compared with those at low redshift. Using a complete sample of nine z > 5.7 quasars, we find that the density of quasars with M ₁₄₅₀ < -26.7 at z ∼ 6 is (6 ± 2) × 10 ^-10 Mpc ^-3 (H ₀ = 65 km s ^-1 Mpc ^-1, Ω = 0.35, and Γ = 0.65), consistent with our previous estimates. The luminosity distribution of the sample is fitted with a power-law luminosity function ψ(L) ∝ L ^-3.2±0.7, somewhat steeper than but consistent with our previous estimates.