Multi-wavelength properties of three new radio-powerful z ∼ 5.6 quasi-stellar objects discovered from RACS

¹ INAF, Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
² SKA Observatory, Science Operations Centre, CSIRO ARRC, 26 Dick Perry Avenue, Kensington, WA 6151, Australia
³ CSIRO Space & Astronomy, PO Box 1130 Bentley, WA 6102, Australia
⁴ David A. Dunlap Department of Astronomy & Astrophysics, University of Toronto, 50 St. George St., Toronto, Ontario M5S 3H4, Canada
⁵ Dunlap Institute for Astronomy & 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 Australian Astronomical Observatory, 105 Delhi Road, North Ryde, NSW 2113, Australia
⁸ Department of Physics and Astronomy, Macquarie University, NSW 2109, Australia
⁹ Australian Research Council Centre of Excellence for All-Sky Astrophysics in 3 Dimensions (ASTRO 3D), Canberra, Australia
¹⁰ INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, I-20126 Milano, Italy
¹¹ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, I-23807 Merate, Italy
¹² International Centre for Radio Astronomy Research, Curtin University, 1 Turner Avenue, Bentley, WA 6102, Australia
¹³ Shanghai Astronomical Observatory, Chinese Academy of Sciences (CAS), 80 Nandan Road, Shanghai 200030, China
¹⁴ School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, China
¹⁵ Key Laboratory of Radio Astronomy and Technology, CAS, A20 Datun Road, Beijing 100101, PR China
¹⁶ Max Planck Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
¹⁷ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129 Bologna, Italy
¹⁸ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica (IASF), Via A. Corti 12, 20133 Milano, Italy

^⋆ Corresponding author; luca.ighina@inaf.it

Received: 4 July 2024
Accepted: 4 November 2024

Abstract

We present a multi-wavelength study of three new z ∼ 5.6 quasi-stellar objects (QSOs) identified from dedicated spectroscopic observations. The three sources were selected as high-z candidates based on their radio and optical/near-infrared properties as reported in the Rapid ASKAP Continuum Survey (RACS), the Dark Energy Survey (DES), and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) survey. These are among the most radio-bright QSOs currently known at z > 5.5, relative to their optical luminosity, having R = S_5GHz/S_4400Å > 100. In this work, we present their identification, and we also discuss their multi-wavelength properties (from the radio to the X-ray band) based on detections in public surveys as well as in dedicated radio and X-ray observations. The three sources present a wide range of properties in terms of relative intensity and spectral shape, highlighting the importance of multi-wavelength observations in accurately characterising these high-z objects. In particular, from our analysis we found one source at z = 5.61 that presents clear blazar properties (strong radio and X-ray emission), making it one of the most distant currently known in this class. Moreover, from the fit of the optical/near-infrared photometric measurements with an accretion disc model as well as the analysis of the CIV broad emission line in one case, we were able to estimate the mass and accretion rate of the central black holes in these systems, finding M_BH ∼ 1 − 10 × 10⁹ M_⊙ accreting at a rate λ_Edd ∼ 0.1 − 0.4. The multi-wavelength characterisation of radio QSOs at z > 5.5, such as the ones reported here, is essential to constraining the evolution of relativistic jets and supermassive black holes hosted in this class of objects.