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 z ≃ 6.5 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 z > 6.5 quasars, corresponding to an age of the universe of <850 Myr, selected as z-band dropouts in the Pan- STARRS1 survey. This increases the number of known z > 6.5 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 M₁₄₅₀ = -27.4. 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 M⊙ and are accreting close to the Eddington limit (L_Bol L_Edd = 0.13 - 1.2). We investigate the ionized regions around the quasars and find near-zone radii of R_NZ = 1.5 - 5.2 proper Mpc, confirming the trend of decreasing near-zone sizes with increasing redshift found for quasars at 5.7 < z < 6.4. By combining RNZ of the PS1 quasars with those of 5.7 < z < 7.1 quasars in the literature, we derive a luminosity-corrected redshift evolution of R_NZ,corrected = (7.2 ± 0.2) - (6.1 ± 0.7) × (z - 6) 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.