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

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 (L1.4a GHz = 0.09-1.0×1034a erga s-1a 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 (f0.2-2.3a keV~1.3×10-13a erga s-1a 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 Ca IV and Mga II broad emission lines. Their masses (log[MBH,MgII] = 8.58-9.14a M⊙) and Eddington ratios (λEdd,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 9000a kma s-1 between the Ca IV and Mga II emission lines. This means that it is one of the most extreme Ca 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 47a 000a kma s-1.