Comprehensive view on a $z\sim6.5$ radio-loud QSO: from the radio to the optical/NIR to the X-ray band

TL;DR

This study presents a comprehensive multi-wavelength analysis of a distant radio-loud quasar at redshift 6.44, revealing its jet properties, black hole mass, and accretion rate, with implications for black hole formation theories.

Contribution

It provides the first detailed multi-wavelength characterization of a z=6.44 radio-loud quasar, including jet properties and black hole mass estimates, challenging existing formation models under certain assumptions.

Findings

Jets are likely young and misaligned from our line of sight.

Radio spectrum shows a turnover at ~650 MHz, not due to self-absorption.

Black hole mass estimated at ~8×10^8 M_sun with high accretion rate.

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 (RL) quasi stellar object (QSO), one of the most distant currently known in this class. The work is based on newly obtained (uGMRT, ATCA, Chandra) as well as archival (GNIRS and X-Shooter) dedicated observations that have not been published yet. 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 $\nu_{\rm peak} \sim 650$ MHz which is unlikely to be associated with self-synchrotron absorption. From the NIR spectrum we derived the mass of the central black hole, M$_{\rm BH}=8.1^{+6.8}_{-5.6} \times 10^8 {\rm M_{\odot}}$, and the Eddington ratio, $\lambda_{\rm EDD} = 0.8^{+0.8}_{-0.6}$, 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 $\sim$8$\times$10$^8$ M$_\odot$ black hole at $z=6.44$ can be explained by a seed black hole ($\sim$10$^{4}$ M$_\odot$) that formed at $z\sim25$, assuming a radiative efficiency $\eta_{\rm d}\sim0.1$. However, by assuming $\eta_{\rm d}\sim0.3$, as expected for jetted systems, the mass observed would challenge current theoretical models of black hole formation.