Physical properties of 15 quasars at $z\gtrsim 6.5$

Abstract

Quasars are galaxies hosting accreting supermassive black holes; due to their brightness, they are unique probes of the early universe. To date, only few quasars have been reported at $z>6.5$ ($<$800 Myr after the Big Bang). In this work, we present six additional $z\gtrsim 6.5$ quasars discovered using the Pan-STARRS1 survey. We use a sample of 15 $z\gtrsim 6.5$ quasars to perform a homogeneous and comprehensive analysis of this highest-redshift quasar population. We report four main results: (1) the majority of $z\gtrsim$6.5 quasars show large blueshifts of the broad CIV 1549Åemission line compared to the systemic redshift of the quasars, with a median value $\sim$3$\times$ higher than a quasar sample at $z\sim$1; (2) we estimate the quasars' black hole masses (M${}_{\mathrm{BH}}\sim$0.3$-$5 $\times$ 10${}^9$ M${}_{\odot }$) via modeling of the MgII 2798Åemission line and rest-frame UV continuum; we find that quasars at high redshift accrete their material (with $⟨(L_{\mathrm{bol}}/L_{\mathrm{Edd}})⟩=0.39$) 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 FeII/MgII abundance ratio with cosmic time; (4) we derive near zone sizes; together with measurements for $z\sim 6$ quasars from recent work, we confirm a shallow evolution of the decreasing quasar near zone sizes with redshift. Finally, we present new millimeter observations of the [CII] 158 $\mu$m emission line and underlying dust continuum from NOEMA for four quasars, and provide new accurate redshifts and [CII]/infrared luminosities estimates. The analysis presented here shows the large range of properties of the most distant quasars.

29 pages, 18 figures; accepted by ApJ

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