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 \simeq 6.5 very faint in the optical z -band , thus locating quasars at higher redshifts require 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 4 to 7 .
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 M _ { 1450 } = -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 \times 10 ^ { 8 } and 4 \times 10 ^ { 9 } M _ { \sun } , and are accreting close to the Eddington limit ( L _ { \mathrm { Bol } } / L _ { \mathrm { Edd } } = 0.13 - 1.2 ) .
We investigate the ionized regions around the quasars and find near zone radii of R _ { \mathrm { 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 R _ { \mathrm { NZ } } 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 _ { \mathrm { NZ,corrected } } = ( 7.2 \pm 0.2 ) - ( 6.1 \pm 0.7 ) \times ( z - 6 ) Mpc .
However , the large spread in R _ { \mathrm { NZ } } 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 .