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

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 $\nu L_{1.4GHz}=10^{41.5}-10^{44.4}$ 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., $-0.5<{\alpha }_r<0.5$). 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.

22 pages plus 6 of appendices. 16 figures and 8 tables in total. Accepted for publication on A&A the 7th of April 2025

References in corpus (73)

• HI4PI: A full-sky HI survey based on EBHIS and GASS
• Cosmology at Low Frequencies: The 21 cm Transition and the High-Redshift Universe
• WSClean: an implementation of a fast, generic wide-field imager for radio astronomy
• The LOFAR Two-metre Sky Survey - I. Survey Description and Preliminary Data Release
• GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey I: A low-frequency extragalactic catalogue
• The LOFAR Two-metre Sky Survey -- V. Second data release
• Distant Radio Galaxies and their Environments
• The Pan-STARRS1 distant z>5.6 quasar survey: more than 100 quasars within the first Gyr of the universe
• The SRG/eROSITA all-sky survey: First X-ray catalogues and data release of the western Galactic hemisphere
• The Dark Energy Survey Data Release 2
• The CatWISE2020 Catalog
• The Rapid ASKAP Continuum Survey I: Design and First Results
• The origins of massive black holes
• The LOFAR Two Meter Sky Survey: Deep Fields, I -- Direction-dependent calibration and imaging
• A Survey of z~6 Quasars in the SDSS Deep Stripe: I. a Flux-Limited Sample at z_{AB}<21
• The Rapid ASKAP Continuum Survey Paper II: First Stokes I Source Catalogue Data Release
• The identification of z-dropouts in Pan-STARRS1: three quasars at 6.5<z<6.7
• Discovery of eight z ~ 6 quasars from Pan-STARRS1
• Constraining the radio-loud fraction of quasars at z>5.5
• Three new VHS-DES Quasars at 6.7 < z < 6.9 and Emission Line Properties at z > 6.5
• LCDM predictions for galaxy protoclusters I: the relation between galaxies, protoclusters and quasars at z~6
• Two bright z > 6 quasars from VST ATLAS and a new method of optical plus mid-infra-red colour selection
• The discovery of a highly accreting, radio-loud quasar at z=6.82
• A survey of luminous high-redshift quasars with SDSS and WISE II. the bright end of the quasar luminosity function at z ~ 5
• The Rapid ASKAP Continuum Survey IV: continuum imaging at 1367.5 MHz and the first data release of RACS-mid
• On the nature of bright compact radio sources at z>4.5
• SRG/eROSITA uncovers the most X-ray luminous quasar at z>6
• The Pan-STARRS1 z>5.6 quasar survey II: Discovery of 55 Quasars at 5.6<z<6.5
• Parsec-scale properties of the radio brightest jetted AGN at z > 6
• Jet enhanced accretion growth of supermassive black holes
• First constraints on the AGN X-ray luminosity function at $z\sim 6$ from an eROSITA-detected quasar
• Discovery of 24 radio-bright quasars at $4.9\le z\le 6.6$ using low-frequency radio observations
• DESI z >~ 5 Quasar Survey. I. A First Sample of 400 New Quasars at z ~ 4.7-6.6
• Radio Detection of VIK J2318$-$3113, the Most Distant Radio-Loud Quasar ($z$=6.44)
• The space density of z>4 blazars
• Enhanced X-ray Emission from the Most Radio-Powerful Quasar in the Universe's First Billion Years
• High-redshift quasars at $z\ge 3$ -- I. Radio spectra
• Random Forests as a viable method to select and discover high redshift quasars
• Discovery of the most X-ray luminous quasar SRGE J170245.3+130104 at redshift z$\approx 5.5$
• XMM-Newton observations of the extremely X-ray luminous quasar CFHQS J142952+544717=SRGE J142952.1+544716 at redshift z=6.18
• The evolution of the heaviest super-massive black-holes in jetted AGNs
• Ultra-luminous quasars at redshift $z>4.5$ from SkyMapper
• Direct observation of an extended X-ray jet at $z$=6.1
• The radio spectral turnover of radio-loud quasars at $z>5$
• A blazar in the epoch of reionization
• New Radio-Loud QSOs at the end of the Re-ionisation Epoch
• Blazar nature of high-z radio-loud quasars
• SIPGI: an interactive pipeline for spectroscopic data reduction
• The GLEAMing of the First Supermassive Black Holes
• The central engine of the highest redshift blazar
• Constraining the Radio Properties of the $z$=6.44 QSO VIK J2318$-$3113
• The SRG/eROSITA All-Sky Survey: X-ray beacons at late cosmic dawn
• Observations by GMRT at 323 MHz of radio-loud quasars at $z>5$
• GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey III: South Galactic Pole data release
• Paving the Way for Euclid and JWST via Optimal Selection of High-z Quasars
• Chandra Detection of Three X-ray Bright Quasars at z>5
• Radio Emission from the First Quasars at $z\sim$ 6-15
• Radio-loud fraction of z>6 quasars
• A powerful (and likely young) radio-loud quasar at z=5.3
• High-redshift quasars at $z\ge 3$: radio variability and MPS/GPS candidates
• Prospects for Observing High-redshift Radio-loud Quasars in the SKA Era: Paving the Way for 21-cm Forest Observations
• Multi-wavelength properties of three new radio-powerful $z\sim 5.6$ quasi-stellar objects discovered from RACS
• Selection of powerful radio galaxies with machine learning
• Comprehensive view on a $z\sim 6.5$ radio-loud QSO: from the radio to the optical/NIR to the X-ray band
• No strong radio absorption detected in the low-frequency spectra of radio-loud quasars at z > 5.6
• Is the X-ray bright z = 5.5 quasar SRGE J170245.3+130104 a blazar?
• Plausible association of distant late M dwarfs with low-frequency radio emission
• Monster radio jet (>66 kpc) observed in quasar at z$\sim$5
• Subarcsecond view on the high-redshift blazar GB 1508+5714 by the International LOFAR Telescope
• A Wide and Deep Exploration of Radio Galaxies with Subaru HSC (WERGS). IX. The Most Overdense Region at z~5 Inhabited by a Massive Radio Galaxy
• Prospects of a statistical detection of the 21-cm forest and its potential to constrain the thermal state of the neutral IGM during reionization
• Structure, spectra and variability of some GPS radio sources
• Milliarcsecond-scale radio structure of the most distant BL Lac object candidate at redshift 6.57

Cited by in corpus (4)

• Discovery and characterization of 25 new quasars at 4.6 < z < 6.9 from wide-field multi-band surveys
• Milliarcsecond-resolution Radio-imaging Survey of Blazar Candidates at 4 < z < 5.4
• Building a Radio AGN Sample from Cosmic Morning -- The Radio High-Redshift Quasar Catalog (RHzQCat): I. Catalog from SDSS Quasars and Radio Surveys at $z>3$
• 4MOST ChANGES: Catalog of high-redshift quasar candidates (4.5 < $z$ < 7) selected with SED fitting