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

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117, Heidelberg, Germany
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti 93/3, I-40129, Bologna, Italy
³ Fakultät für Physik und Astronomie, Universität Heidelberg, Im Neuenheimer Feld 226, 69120 Heidelberg, Germany
⁴ Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands
⁵ Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA
⁶ Department of Astronomy, University of Michigan, 1085 S. University Ave., Ann Arbor, MI 48109, USA
⁷ Dipartimento di Fisica, Università di Trieste, Sezione di Astronomia, Via G.B. Tiepolo 11, I-34131 Trieste, Italy
⁸ INAF–Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, I-34131 Trieste, Italy
⁹ Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan
¹⁰ Center for Data-Driven Discovery, Kavli IPMU (WPI), UTIAS, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan
¹¹ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China
¹² Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
¹³ Millennium Institute of Astrophysics (MAS), Nuncio Monseñor Sótero Sanz 100, Providencia, Santiago, Chile
¹⁴ Núcleo de Astronomía de la Facultad de Ingeniería, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
¹⁵ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany
¹⁶ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91109, USA
¹⁷ International Gemini Observatory/NSF NOIRLab, 670 N A’ohoku Place, Hilo, Hawai’i 96720, USA

^⋆ Corresponding author.

Received: 29 March 2025
Accepted: 19 May 2025

Abstract

Luminous quasars at z>4 provide key insights into the early Universe. Their rarity necessitates wide-field multiband 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, which we complemented by literature-based quasar candidate catalogs. We report the discovery of 25 new quasars at 4.6<z<6.9 (six at z≥6.5), with M₁₄₅₀ 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 GHz = 0.09−1.0×10³⁴ erg s⁻¹ Hz⁻¹). One source at z = 4.71 exhibits typical blazar-like properties, including a flat radio spectrum, a radio loudness of ∼1000, and multifrequency variability. It was also detected by SRG/eROSITA X-ray telescope (f_{0.2−2.3 keV}∼1.3×10⁻¹³ erg s⁻¹ cm⁻²). In addition, for seven 6.3<z<6.9 quasars, we present near-infrared spectroscopy and estimated the central black hole mass from their C IV and Mg II broad emission lines. Their masses (log[M_BH,MgII] = 8.58−9.14 M_⊙) and Eddington ratios (λ_Edd,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 IV and Mg II emission lines. This means that it is one of the most extreme C IV outflows currently known. The sample also includes three high-ionization broad absorption line (HiBAL) quasars. One of these quasars shows potential evidence of an extremely fast outflow feature that reaches 47 000 km s⁻¹.