A survey of z>5.7 quasars in the Sloan Digital Sky Survey.: IV.: Discovery of seven additional quasars

We present the discovery of seven quasars at z > 5.7, selected from similar to 2000 deg(2) of multicolor imaging data of the Sloan Digital Sky Survey. The new quasars have redshifts z from 5.79 to 6.13. Five are selected as part of a complete flux-limited sample in the SDSS northern Galactic cap; two have larger photometric errors and are not part of the complete sample. One of the new quasars, SDSS J1335+3533 (z = 5.93), exhibits no emission lines; the 3 sigma limit on the rest-frame equivalent width of the Ly alpha+N v line is 5 angstrom. It is the highest redshift lineless quasar known and could be a gravitational lensed galaxy, a BL Lac object, or a new type of quasar. Two new z > 6 quasars, SDSS 1250+3130 (z = 6.13) and SDSS J1137+3549 (z = 6.01), show deep Gunn-Peterson absorption gaps in Ly alpha. These gaps are narrower than the complete Gunn-Peterson absorption troughs observed among quasars at z > 6.2 and do not have complete Ly alpha absorption.