Discovery of 24 radio-bright quasars at 4.9 ≤ z ≤ 6.6 using low-frequency radio observations^⋆,⋆⋆

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: gloudemans@strw.leidenuniv.nl
² Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
³ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁴ Institute for Cosmic Ray Research, the University of Tokyo, 5-1-5 Kashiwa-no-Ha, Kashiwa City, Chiba 277-8582, Japan
⁵ McDonald Observatory, University of Texas at Austin, 2515 Speedway, Stop C1402, Austin, TX 78712, USA
⁶ Department of Astronomy, University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712, USA
⁷ Hobby Eberly Telescope, University of Texas, Austin, TX 78712, USA
⁸ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁹ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
¹⁰ Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
¹¹ Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA
¹² European Southern Observatory, Karl-Schwarzschild str. 2, 85748 Garching bei München, Germany
¹³ INAF-IAPS, Via del Fosso del Cavaliere 100, 00133 Rome, Italy
¹⁴ ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
¹⁵ Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 18 August 2022
Accepted: 27 September 2022

Abstract

High-redshift quasars (z >  5) that also shine brightly at radio wavelengths are unique signposts of supermassive black hole activity in the early universe. However, bright radio sources at z ≥ 5 are extremely rare and therefore we have started a campaign to search for new high-z quasars by combining an optical dropout selection driven by the g, r, and z bands from the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys with low-frequency radio observations from the LOFAR Two-metre Sky Survey (LoTSS). Currently, LoTSS covers a large fraction of the northern sky (∼5720 deg²) to such a depth (median noise level ∼83 μJy beam⁻¹) that about 30% of the general quasar population is detected − which is a factor of 5–10 more than previous large sky radio surveys such as NVSS and FIRST, respectively. In this paper, we present the discovery of 20 new quasars (and the independent confirmation of four) between 4.9 ≤ z ≤ 6.6. Out of the 24 quasars, 21 satisfy the traditional radio-loudness criterion of R = f_5 GHz/f_4400 Å >  10, with the full sample spanning R ∼ 6–1000, thereby more than doubling the sample of known radio-loud quasars at z ≥ 5. Our radio detection requirement strongly decreases the contamination of stellar sources and allows one to select these quasars in a broad redshift range. Despite selecting our quasar candidates using fewer and less conservative colour restrictions, both the optical and near-infrared colours, Lyα emission line properties, and dust reddening, E(B − V), measurements of our quasar sample do not deviate from the known radio-quiet quasar population, suggesting similar optical quasar properties of the radio-loud and radio-quiet quasar population at high-z. Our campaign demonstrates the potential for discovering new high-z quasar populations through next generation radio continuum surveys.