Exploring the radio spectral energy distribution of the ultraluminous radio-quiet quasar SDSS J0100+2802 at redshift 6.3

TL;DR

This study uses deep VLA observations to analyze the radio spectral energy distribution of a high-redshift, radio-quiet quasar, revealing a compact core and diffuse emission dominated by the active galactic nucleus.

Contribution

First detailed radio spectral analysis of a z=6.3 radio-quiet quasar, identifying core and diffuse emission components and their origins.

Findings

Radio spectral index of -0.52±0.18

Detection of variability in flux density and spectral index

Diffuse emission exceeds star formation expectations

Abstract

We report deep Karl G. Jansky Very Large Array (VLA) observations of the optically ultraluminous and radio-quiet quasar SDSS J010013.02 + 280225.8 (hereafter J0100+2802) at redshift $z=$6.3. We detected the radio continuum emission at 1.5 GHz, 6 GHz, and 10 GHz. This leads to a radio power-law spectral index of $\alpha = -0.52\pm0.18$ ($S \propto \nu^{\alpha}$). The radio source is unresolved in all VLA bands with an upper limit to the size of $0.2^{\prime \prime}$ (i.e., $\sim$ 1.1 kpc) at 10 GHz. We find variability in the flux density (increase by $\sim 33\%$) and the spectral index (steepened) between observations in 2016 and 2017. We also find that the VLA 1.5 GHz flux density observed in the same year is 1.5 times that detected with the Very Long Baseline Array (VLBA) in 2016 at the same frequency. This difference suggests that half of the radio emission from J0100+2802 comes from a compact core within 40 pc, and the rest comes from the surrounding few kpc area which is diffuse and resolved out in the VLBA observations. The diffuse emission is four times brighter than that would be expected if driven by star formation. We conclude that the central active galactic nucleus is the dominant power engine of the radio emission in J0100+2802.