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 similar to1700 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 greater than or similar to 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 similar to 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 similar to 6 is (6 +/- 2) x 10(-10) Mpc(-3) (H-0 = 65 km s(-1) Mpc(-1), Omega = 0.35, and Lambda = 0.65), consistent with our previous estimates. The luminosity distribution of the sample is fitted with a power-law luminosity function Psi( L) proportional to L-3.2 +/- 0.7, somewhat steeper than but consistent with our previous estimates.