Candidate type II quasars from the Sloan Digital Sky Survey.: I.: Selection and optical properties of a sample at 0.3 < Z < 0.83

Type II quasars are the long-sought luminous analogs of type 2 (narrow emission line) Seyfert galaxies, suggested by unification models of active galactic nuclei (AGNs) and postulated to account for an appreciable fraction of the cosmic hard X-ray background. We present a sample of 291 type II AGNs at redshifts 0.3<Z<0.83 from the spectroscopic data of the Sloan Digital Sky Survey. These objects have narrow (FWHM<2000 km s(-1)), high equivalent width emission lines with high-ionization line ratios. We describe the selection procedure and discuss the optical properties of the sample. About 50% of the objects have [O III] λ5008 line luminosities in the range 3x10(8)-10(10) L-⊙, comparable to those of luminous (-27<M-B<-23) quasars; this, along with other evidence, suggests that the objects in the luminous subsample are type II quasars.