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.

arXiv comment22 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 (141)

• LSST: from Science Drivers to Reference Design and Anticipated Data Products
• Observational Evidence of AGN Feedback
• Composite Quasar Spectra From the Sloan Digital Sky Survey
• Overview of the DESI Legacy Imaging Surveys
• HI4PI: A full-sky HI survey based on EBHIS and GASS
• Cosmology at Low Frequencies: The 21 cm Transition and the High-Redshift Universe
• Galaxy and Mass Assembly (GAMA): survey diagnostics and core data release
• A Survey of z>5.8 Quasars in the Sloan Digital Sky Survey I: Discovery of Three New Quasars and the Spatial Density of Luminous Quasars at z~6
• WSClean: an implementation of a fast, generic wide-field imager for radio astronomy
• Spectral Energy Distributions of Hard X-ray selected AGNs in the XMDS Survey
• The GMRT 150 MHz All-sky Radio Survey: First Alternative Data Release TGSS ADR1
• Relativistic Jets in Active Galactic Nuclei
• SUMSS: A Wide-Field Radio Imaging Survey of the Southern Sky. II. The Source Catalogue
• A Survey of z>5.7 Quasars in the Sloan Digital Sky Survey II: Discovery of Three Additional Quasars at z>6
• Automated data reduction workflows for astronomy
• The LOFAR Two-metre Sky Survey - I. Survey Description and Preliminary Data Release
• The Connection between X-ray- and Radio-Selected BL Lacertae Objects
• The LOFAR Two-metre Sky Survey - II. First 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 Cosmic Lens All-Sky Survey: I. Source selection and observations
• 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 realm of the galaxy protoclusters
• The Dark Energy Survey Data Release 2
• The CatWISE2020 Catalog
• GLEAM: The GaLactic and Extragalactic All-sky MWA survey
• Discovery of three z>6.5 quasars in the VISTA Kilo-degree Infrared Galaxy (VIKING) survey
• The Rapid ASKAP Continuum Survey I: Design and First Results
• Radio galaxies and feedback from AGN jets
• The origins of massive black holes
• The LOFAR Two Meter Sky Survey: Deep Fields, I -- Direction-dependent calibration and imaging
• The Phase II Murchison Widefield Array: Design Overview
• Compact steep-spectrum and peaked-spectrum radio sources
• The tight relation between X-ray and ultraviolet luminosity of quasars
• Subaru high-z exploration of low-luminosity quasars (SHELLQs). I. Discovery of 15 quasars and bright galaxies at 5.7 < z < 6.9
• 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
• Exploring Reionization-Era Quasars III: Discovery of 16 Quasars at $6.4\lesssim z\lesssim 6.9$ with DESI Legacy Imaging Surveys and UKIRT Hemisphere Survey and Quasar Luminosity Function at $z\sim 6.7$
• The identification of z-dropouts in Pan-STARRS1: three quasars at 6.5<z<6.7
• High-Redshift Quasars Found in Sloan Digital Sky Survey Commissioning Data
• The LOFAR window on star-forming galaxies and AGN - curved radio SEDs and IR-radio correlation at $0<z<2.5$
• Spectral ageing in the lobes of FR-II radio galaxies: New methods of analysis for broadband radio data
• HI 21cm absorption beyond the epoch of re-ionization
• Discovery of eight z ~ 6 quasars from Pan-STARRS1
• Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). II. Discovery of 32 Quasars and Luminous Galaxies at 5.7 < z < 6.8
• The CatWISE Preliminary Catalog: Motions from $WISE$ and $NEOWISE$ Data
• Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). IV. Discovery of 41 Quasars and Luminous Galaxies at 5.7 < z < 6.9
• Eight new luminous z > 6 quasars selected via SED model fitting of VISTA, WISE and Dark Energy Survey Year 1 Observations
• Radio Variability of Radio Quiet and Radio Loud Quasars
• CubiCal - Fast radio interferometric calibration suite exploiting complex optimisation
• The Discovery of a High-redshift Quasar without Emission Lines from Sloan Digital Sky Survey Commissioning Data
• Exploring Reionization-Era Quasars IV: Discovery of Six New $z\gtrsim 6.5$ Quasars with DES, VHS and unWISE Photometry
• The FLAMINGOS Extragalactic Survey
• Constraining the radio-loud fraction of quasars at z>5.5
• Ionization near-zones associated with quasars at z ~ 6
• Three new VHS-DES Quasars at 6.7 < z < 6.9 and Emission Line Properties at z > 6.5
• High Frequency Peakers: young radio sources or flaring blazars?
• LCDM predictions for galaxy protoclusters I: the relation between galaxies, protoclusters and quasars at z~6
• A survey of luminous high-redshift quasars with SDSS and WISE. I. target selection and optical spectroscopy
• X-ray Lighthouses of the High-Redshift Universe. II. Further Snapshot Observations of the Most Luminous z>4 Quasars with Chandra
• The X-ray properties of z$\sim$6 luminous quasars
• The first blazar observed at z>6
• Discovery of a z=6.1 Radio-Loud Quasar in the NDWFS
• Two bright z > 6 quasars from VST ATLAS and a new method of optical plus mid-infra-red colour selection
• A Sample of Ultra Steep Spectrum Sources Selected from the Westerbork In the Southern Hemisphere (WISH) survey
• 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
• DES J0454-4448: Discovery of the First Luminous z > 6 Quasar from the Dark Energy Survey
• CAPTURE: A continuum imaging pipeline for the uGMRT
• The Faint End of the Quasar Luminosity Function at $z\sim 5$ from the Subaru Hyper Suprime-Cam Survey
• Physical properties in young radio sources. VLBA observations of high frequency peaking radio sources
• A powerful radio-loud quasar at the end of cosmic reionization
• The role of relativistic jets in the heaviest and most active supermassive black holes at high redshift
• The Rapid ASKAP Continuum Survey IV: continuum imaging at 1367.5 MHz and the first data release of RACS-mid
• High-Redshift Quasars Found in Sloan Digital Sky Survey Commissioning Data II: The Spring Equatorial Stripe
• The First High Redshift Quasar from Pan-STARRS
• 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
• An X-ray and Multiwavelength Survey of Highly Radio-Loud Quasars at z > 4: Jet-Linked Emission in the Brightest Radio Beacons of the Early Universe
• Discovery of 16 New z ~ 5.5 Quasars : Filling in the Redshift Gap of Quasar Color Selection
• SDSS J102623.61+254259.5: the second most distant blazar at z=5.3
• 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
• The space density of z>4 blazars
• Radio Detection of VIK J2318$-$3113, the Most Distant Radio-Loud Quasar ($z$=6.44)
• X-ray properties of z>4 blazars
• Discovery of a galaxy overdensity around a powerful, heavily obscured FRII radio galaxy at z=1.7: star formation promoted by large-scale AGN feedback?
• SDSS J114657.79+403708.6: the third most distant blazar at z=5.0
• Prospects for detecting the 21cm forest from the diffuse intergalactic medium with LOFAR
• Radio-Selected Quasars in the Sloan Digital Sky Survey
• Enhanced X-ray Emission from the Most Radio-Powerful Quasar in the Universe's First Billion Years
• Accretion Discs in Blazars
• High-redshift quasars at $z\ge 3$ -- I. Radio spectra
• Extragalactic megahertz-peaked spectrum radio sources at milliarcsecond scales
• LOFAR VLBI Studies at 55 MHz of 4C 43.15, a z=2.4 Radio Galaxy
• Discovery of the most X-ray luminous quasar SRGE J170245.3+130104 at redshift z$\approx 5.5$
• Random Forests as a viable method to select and discover high redshift quasars
• XMM-Newton observations of the extremely X-ray luminous quasar CFHQS J142952+544717=SRGE J142952.1+544716 at redshift z=6.18
• Parsec-scale morphology and spectral index distribution in faint high frequency peakers
• Ultra-luminous quasars at redshift $z>4.5$ from SkyMapper
• Direct observation of an extended X-ray jet at $z$=6.1
• The evolution of the heaviest super-massive black-holes in jetted AGNs
• The radio spectral turnover of radio-loud quasars at $z>5$
• Blazar nature of high-z radio-loud quasars
• A blazar in the epoch of reionization
• New Radio-Loud QSOs at the end of the Re-ionisation Epoch
• An extremely X--ray weak blazar at z=5
• SIPGI: an interactive pipeline for spectroscopic data reduction
• The GLEAMing of the First Supermassive Black Holes
• The central engine of the highest redshift blazar
• Mr-Moose: An advanced SED-fitting tool for heterogeneous multi-wavelength datasets
• The SRG/eROSITA All-Sky Survey: X-ray beacons at late cosmic dawn
• Constraining the Radio Properties of the $z$=6.44 QSO VIK J2318$-$3113
• Observations by GMRT at 323 MHz of radio-loud quasars at $z>5$
• Paving the Way for Euclid and JWST via Optimal Selection of High-z Quasars
• Radio Emission from the First Quasars at $z\sim$ 6-15
• Chandra Detection of Three X-ray Bright Quasars at z>5
• GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey III: South Galactic Pole data release
• 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
• No strong radio absorption detected in the low-frequency spectra of radio-loud quasars at z > 5.6
• Comprehensive view on a $z\sim 6.5$ radio-loud QSO: from the radio to the optical/NIR to the X-ray band
• 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
• Is the X-ray bright z = 5.5 quasar SRGE J170245.3+130104 a blazar?
• X-ray constraints on the spectral energy distribution of the $z=5.18$ blazar SDSS J013127.34-032100.1
• Plausible association of distant late M dwarfs with low-frequency radio emission
• Monster radio jet (>66 kpc) observed in quasar at z$\sim$5
• 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