High-z radio quasars in RACS

I. Selection, identification, and multi-wavelength properties

¹ INAF, Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milano, Italy
² Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
³ SKA Observatory, Science Operations Centre, CSIRO ARRC, 26 Dick Perry Avenue, Kensington, WA 6151, Australia
⁴ David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada
⁵ Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada
⁶ Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
⁷ INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, I-20126 Milano, Italy
⁸ Como Lake Centre for AstroPhysics (CLAP), DiSAT, Università dell’Insubria, Via Valleggio 11, 22100 Como, Italy
⁹ International Centre for Radio Astronomy Research, Curtin University, 1 Turner Avenue, Bentley, WA 6102, Australia
¹⁰ Instituto de Astrofísica e Ciências do Espaço, Universidade de Lisboa, OAL, Tapada da Ajuda, Lisboa, Portugal
¹¹ Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Lisbon, Portugal
¹² Dipartimento di Fisica e Astronomia, Alma Mater Studiorum, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
¹³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Piero Gobetti 93/3, 40129 Bologna, Italy
¹⁴ Shanghai Astronomical Observatory, Chinese Academy of Sciences (CAS), 80 Nandan Road, Shanghai 200030, China
¹⁵ School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, China
¹⁶ Key Laboratory of Radio Astronomy and Technology, CAS, A20 Datun Road, Beijing 100101, PR China
¹⁷ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica (IASF), Via A. Corti 12, 20133 Milano, Italy
¹⁸ INAF – Istituto di Radioastronomia, Via Gobetti 101, I-40129 Bologna, Italy
¹⁹ School of Mathematical and Physical Sciences, Faculty of Science and Engineering, Macquarie University, Sydney, NSW 2109, Australia
²⁰ Australian Research Council Centre of Excellence for All-Sky Astrophysics in 3 Dimensions (ASTRO 3D), Canberra, Australia
²¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate, Italy

^⋆ Corresponding author: luca.ighina@cfa.harvard.edu

Received: 31 December 2024
Accepted: 7 April 2025

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.