The radio-loud fraction of quasars at z > 6

Quasars at redshifts $z>6$ are an excellent probe of the formation and evolution of supermassive black holes in the early Universe. The population of radio-luminous quasars is of particular interest, as such quasars could potentially be used to study the neutral intergalactic medium during cosmic reionisation via H$\,$I 21$\,$cm absorption studies. However, the lack of deep radio observations of $z>6$ quasars leaves the population poorly constrained, and suitable candidates for an H$\,$I 21$\,$cm absorption study have yet to be found. In this work, we present Jansky Very Large Array (VLA) 1$-$2 GHz radio continuum observations of 138 quasars at redshifts 6.0 ≤ z<7.6. We detect the radio continuum emission of the $z=6.1$ quasar J1034-1425, with a 1.6 GHz flux density of 170± 36\,μJy. This quasar is radio-quiet with radio-loudness, R ≡ f5~GHz/fν,4400 A = 2.4±0.5. In addition, we detect 7 other quasars at z>6, which have previously been characterised in the literature at these frequencies. Using the full sample, we estimate the radio-loud fraction to be 3.8+6.2-2.4%, where the uncertainties are 95% confidence intervals. This is lower than recent estimates of the radio-loud fraction in the literature, but is still marginally consistent with no redshift evolution of the radio-loud fraction. We explore the undetected quasar population by stacking their continuum images at their optical positions and obtain a median stacked flux density of 13.8± 3.9~μJy and luminosity of logL5~GHz/(W~Hz⁻¹)=24.2±0.1.