The Identification of Z-dropouts in Pan-STARRS1: Three Quasars at 6.5< z< 6.7

Luminous distant quasars are unique probes of the high-redshift intergalactic medium (IGM) and of the growth of massive galaxies and black holes in the early universe. Absorption due to neutral hydrogen in the IGM makes quasars beyond a redshift of very faint in the optical z band, thus locating quasars at higher redshifts requires large surveys that are sensitive above 1 micron. We report the discovery of three new quasars, corresponding to an age of the universe of Myr, selected as z-band dropouts in the Pan-STARRS1 survey. This increases the number of known quasars from four to seven. The quasars have redshifts of z = 6.50, 6.52, and 6.66, and include the brightest z-dropout quasar reported to date, PSO J036.5078 + 03.0498 with . We obtained near-infrared spectroscopy for the quasars, and from the Mg ii line, we estimate that the central black holes have masses between 5 × 108 and 4 × 109 and are accreting close to the Eddington limit (). We investigate the ionized regions around the quasars and find near-zone radii of proper Mpc, confirming the trend of decreasing near-zone sizes with increasing redshift found for quasars at . By combining RNZ of the PS1 quasars with those of quasars in the literature, we derive a luminosity-corrected redshift evolution of Mpc. However, the large spread in RNZ in the new quasars implies a wide range in quasar ages and/or a large variation in the neutral hydrogen fraction along different lines of sight.