Discovery and characterization of 25 new quasars at 4.6 < z < 6.9 from wide-field multi-band surveys

Abstract

Luminous quasars at $z>4$ provide key insights into the early Universe. Their rarity necessitates wide-field multi-band surveys to efficiently separate them from the main astrophysical contaminants (i.e., ultracool dwarfs). To expand the sample of high-$z$ quasars, we conducted targeted selections using optical, infrared, and radio surveys, complemented by literature-based quasar candidate catalogs. In this paper, we report the discovery of \nqsos\ new quasars at $4.6<z<6.9$ (six at $z\ge 6.5$), with $M_{1450}$ between $-$25.4 and $-$27.0. We also present new spectra of six $z>6.5$ quasars we selected, but whose independent discovery has already been published in the literature. Three of the newly discovered quasars are strong radio emitters (L${}_{1.4\mathrm{GHz}}$$=0.09-1.0\times$10${}^{34}$erg s${}^{-1}$ Hz${}^{-1}$). Among them, one source at $z=4.71$ exhibits typical blazar-like properties, including a flat radio spectrum, radio-loudness $\sim$1000, and multi-frequency variability. It is also detected by SRG/eROSITA X-ray telescope (f${}_{0.2-2.3keV}\sim 1.3\times 10^{-13}$erg s${}^{-1}$ cm${}^{-2}$). In addition, for seven $6.3<z<6.9$ quasars we present near-infrared spectroscopy and estimate the central black hole mass from their C$\mathrm{IV}$ and Mg$\mathrm{II}$ broad emission lines.Their masses (log[M${}_{\mathrm{BH},\mathrm{MgII}}$]$=8.58-9.14{\mathrm{M}}_{\odot }$) and Eddington ratios (${\lambda }_{\mathrm{Edd},\mathrm{MgII}}=0.74-2.2$) are consistent with other $z>6$ quasars reported in the literature. A $z=6.3$ quasar exhibits a velocity difference of approximately $9000$ km s${}^{-1}$ between the C$\mathrm{IV}$ and Mg$\mathrm{II}$ emission lines, making it one of the most extreme C$\mathrm{IV}$ outflows currently known. Additionally, the sample includes three high-ionization broad absorption line quasars. One of these quasars shows potential evidence of an extremely fast outflow feature, reaching $48000$ km s${}^{-1}$.

25 pages, 8 figures, 12 tables, Accepted for publication in A&A

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