High-$z$ radio Quasars in RACS I: Selection, identification, and multi-wavelength properties

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, A. López-Sánchez, I. Matute, C. Reynolds, A. Rossi, C. Spingola, P. Severgnini, F. Tavecchio

First listed 2025-04-14 | Last updated 2025-04-14

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 the 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$_{888MHz}>1$ mJy and mag$z<21.3$ over an area of 16000deg$^2$. 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 by 33 powerful quasars, expected to be ~90% complete at mag$z<21.3$ and S$_{888MHz}>1$ mJy. Having rest-frame radio luminosities in the range $νL_{1.4GHz}=10^{41.5}-10^{44.4}$ 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 {\it 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 found 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.

Short digest

Cross-matching RACS-low (888 MHz) with DES and Pan-STARRS, the authors selected 45 bright radio/optical-dropout candidates (S_888MHz>1 mJy, z_mag<21.3) over 16,000 deg^2 and spectroscopically confirmed 24 new quasars, 13 at 4.5<z<5 and 11 at z>5. Including comparable literature objects, the z>5 RACS set reaches 33 powerful radio quasars with νL_1.4GHz≈10^41.5–10^44.4 erg s^-1 and is ~90% complete at the stated flux and magnitude limits. Multi-band radio plus new X-ray coverage (Chandra, uGMRT, MeerKAT, ATCA) reveal predominantly flat spectra (22 with −0.5<α_r<0.5) and X-ray luminosities exceeding coronal expectations. The combined evidence points to many blazar-like, jet-aligned systems, delivering a bright, homogeneous anchor sample for population studies of early SMBH growth.

Key figures to inspect

• Fig. 1 — Map the 16,000 deg^2 footprint and locate which new sources lie at z>5 (squares) versus previously known objects (darker stars) to gauge sky coverage and selection uniformity.
• Fig. 2 — Inspect the DES color/magnitude cuts, especially how the tiered criteria isolate specific redshift windows; note where dropout thresholds sit relative to the radio-prior from RACS.
• Fig. 3 — Use the color–color panels to see contamination control: region (2) shows many interlopers (likely stars with spurious radio matches), and the role of Y-band visual checks in pruning candidates.
• Fig. 4 — Compare the Pan-STARRS selection branches for different redshift bins and how ‘mean’ magnitudes are applied; this sets expectations for completeness versus contamination outside DES coverage.
• Radio/X-ray SED panels (if included in the full text) — Look for single vs. broken power-law radio fits and instances of X-ray excess over coronal predictions that flag likely blazars.