Radio Detection of VIK J2318$-$3113, the Most Distant Radio-Loud Quasar ($z$=6.44)

TL;DR

This paper reports the detection of the most distant radio-loud quasar at redshift 6.44, revealing its high luminosity, potential jet activity, and implications for early supermassive black hole growth.

Contribution

First detection of a radio-loud quasar at z=6.44, providing insights into early black hole activity and jet presence in the young universe.

Findings

Most distant radio-loud quasar observed to date.

Possible jet activity indicated by flux variability.

High black hole mass inferred at early cosmic times.

Abstract

We report the 888 MHz radio detection in the Rapid ASKAP Continuum Survey (RACS) of VIK J2318$-$3113, a $z$=6.44 quasar. Its radio luminosity (1.2 $\times 10^{26}$ W Hz$^{-1}$ at 5 GHz) compared to the optical luminosity (1.8 $\times 10^{24}$ W Hz$^{-1}$ at 4400 A) makes it the most distant radio-loud quasar observed so far, with a radio loudness R$\sim$70 (R$=L_\mathrm{{5GHz}}/L_\mathrm{{4400A}}$). Moreover, the high bolometric luminosity of the source (L$_\mathrm{{bol}}$=7.4 $\times 10^{46}$ erg s$^{-1}$) suggests the presence of a supermassive black hole with a high mass ($\gtrsim$6 $\times$10$^8$ M$_\odot$) at a time when the Universe was younger than a billion years. Combining the new radio data from RACS with previous ASKAP observations at the same frequency, we found that the flux density of the source may have varied by a factor of $\sim$2, which could suggest the presence of a relativistic jet oriented towards the line of sight, that is, a blazar nature. However, currently available radio data do not allow us to firmly characterise the orientation of the source. Further radio and X-ray observations are needed.