L. Ighina, A. Caccianiga, A. Moretti, J. W. Broderick, J. K. Leung, F. Rigamonti, N. Seymour, J. Afonso, T. Connor, C. Vignali, Z. Wang, T. An, B. Arsioli, S. Bisogni, D. Dallacasa, R. Della Ceca, Y. Liu, Á. R. López-Sánchez, I. Matute, C. Reynolds, A. Rossi, C. Spingola, P. Severgnini, F. Tavecchio

Abstract
Radio-bright jetted quasars at z > 5 serve as unique laboratories for studying supermassive black hole activity in the early Universe. In this work, we present a sample of high-z jetted quasars selected from a combination of the radio Rapid ASKAP Continuum Survey (RACS) with deep wide-area optical/near-infrared surveys. From this cross-match, we selected 45 new high-z radio quasar candidates with S_{888 MHz} > 1 mJy and magz < 21.3 over an area of 16 000 deg². Using spectroscopic observations, we confirmed the high-z nature of 24 new quasars, 13 at 4.5 < z < 5, and 11 at z > 5. If we also consider similar, in terms of radio/optical fluxes and sky position, quasars at z > 5 already reported in the literature, the overall z > 5 RACS sample is composed of 33 powerful quasars that are expected to be ∼90% complete at magz < 21.3 and S_{888 MHz} > 1 mJy. Having rest-frame radio luminosities in the range νL_{1.4 GHz} = 10^41.5 − 10^44.4 erg s⁻¹, this sample contains the most extreme radio quasars currently known in the early Universe. We also present all X-ray and radio data currently available for the sample, including new dedicated Chandra, uGMRT, MeerKAT, and ATCA observations for a sub-set of the sources. From the modelling of their radio emission, either with a single power law or a broken power law, we find that these systems have a wide variety of spectral shapes, with most quasars (22) having a flat radio emission (i.e. −0.5 < α_r < 0.5). At the same time, the majority of the sources with X-ray coverage present a high-energy luminosity larger than the one expected from the X-ray corona only. Both the radio and X-ray properties of the high-z RACS sample suggest that many of these sources have relativistic jets oriented close to our line of sight (i.e. blazars) and can therefore be used to perform statistical studies on the entire jetted population at high redshift.

Keywords
galaxies: active / galaxies: high-redshift / galaxies: jets / quasars: general / quasars: supermassive black holes

Astronomy & Astrophysics
Volume 698, Article Number A158, Number of pages 27
2025 June

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