2025A&A...699A.335B

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.