Physical Properties of 15 Quasars at z ≳ 6.5

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 (<800Myr 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 ∼3× higher than a quasar sample at z ∼ 1; (2) we estimate the quasars black hole masses (M_BH ∼(0.3-5) × 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.