Comprehensive view of a z  ∼  6.5 radio-loud quasi-stellar object: From the radio to the optical/NIR to the X-ray band

¹ INAF, Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milano, Italy
e-mail: luca.ighina@inaf.it
² DiSAT, Università degli Studi dell’Insubria, Via Valleggio 11, 22100 Como, Italy
³ International Centre for Radio Astronomy Research, Curtin University, 1 Turner Avenue, Bentley, WA 6102, Australia
⁴ SKA Observatory, Science Operations Centre, CSIRO ARRC, 26 Dick Perry Avenue, Kensington, WA 6151, Australia
⁵ CSIRO Space & Astronomy, PO Box 1130 Bentley, WA 6102, Australia
⁶ Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia
⁷ CSIRO Space and Astronomy, PO Box 76 Epping, NSW 1710, Australia
⁸ ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), Hawthorn, VIC 3122, Australia
⁹ David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto, 50 St. George St., Toronto, Ontario M5S3H4, Canada
¹⁰ Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
¹¹ INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
¹² Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio (OAS), Via Gobetti 93/3, 40129 Bologna, Italy
¹⁴ ATNF, CSIRO Space & Astronomy, PO Box 1130 Bentley, WA 6102, Australia

Received: 28 January 2024
Accepted: 22 April 2024

Abstract

We present a multi-wavelength analysis, from the radio to the X-ray band, of the redshift z = 6.44 VIK J2318−31 radio-loud quasi-stellar object, one of the most distant currently known of this class. The work is based on newly obtained observations (uGMRT, ATCA, and Chandra) as well as dedicated archival observations that have not yet been published (GNIRS and X-shooter). Based on the observed X-ray and radio emission, its relativistic jets are likely young and misaligned from our line of sight. Moreover, we can confirm, with simultaneous observations, the presence of a turnover in the radio spectrum at ν_peak ∼ 650 MHz that is unlikely to be associated with self-synchrotron absorption. From the near-infrared spectrum we derived the mass of the central black hole, $M_{\mathrm{BH}}=8.1_{-5.6}^{+6.8}\times {10}^8{M}_{\odot }$, and the Eddington ratio, ${\lambda }_{\mathrm{EDD}}=0.8_{-0.6}^{+0.8}$, using broad emission lines as well as an accretion disc model fit to the continuum emission. Given the high accretion rate, the presence of a ∼8 × 10⁸ M_⊙ black hole at z = 6.44 can be explained by a seed black hole (∼10⁴ M_⊙) that formed at z ∼ 25, assuming a radiative efficiency η_d ∼ 0.1. However, by assuming η_d ∼ 0.3, as expected for jetted systems, the mass observed would challenge current theoretical models of black hole formation.