2017ApJ...849...91M

Quasars are galaxies hosting accreting supermassive black holes; due to their brightness, they are unique probes of the early universe. To date, only a few quasars have been reported at z 6.5 (<800 Myr after the big bang). In this work, we present six additional z 6.5 quasars discovered using the Pan-STARRS1 survey. We use a sample of 15 z 6.5 quasars to perform a homogeneous and comprehensive analysis of this highest-redshift quasar population. We report four main results: (1) the majority of z 6.5 quasars show large blueshifts of the broad C IV λ1549 emission line compared to the systemic redshift of the quasars, with a median value ∼3x higher than a quasar sample at z∼1; (2) we estimate the quasars' black hole masses (M_BH ∼(0.3-5) x 10⁹ M_☉) via modeling of the Mg II λ2798 emission line and rest-frame UV continuum and find that quasars at high redshift accrete their material (with <(L_bol/L_Edd)>=0.39) at a rate comparable to a luminosity-matched sample at lower redshift, albeit with significant scatter (0.4 dex); (3) we recover no evolution of the Fe II/Mg II abundance ratio with cosmic time; and (4) we derive near-zone sizes and, together with measurements for z∼6 quasars from recent work, confirm a shallow evolution of the decreasing quasar near-zone sizes with redshift. Finally, we present new millimeter observations of the [C II] 158 µm emission line and underlying dust continuum from NOEMA for four quasars and provide new accurate redshifts and [C II]/infrared luminosity estimates. The analysis presented here shows the large range of properties of the most distant quasars.