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 reionization 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 mu Jy. This quasar is radio-quiet with radio-loudness, R equivalent to f(5 GHz)/f(nu, 4400 angstrom) = 2.4 +/- 0.5. In addition, we detect seven other quasars at z > 6, which have previously been characterized in the literature at these frequencies. Using the full sample, we estimate the radio-loud fraction to be 3.8-2.4+6.2 per cent, where the uncertainties are 95 per cent 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 mu Jy and luminosity of log L-5 GHz/(W Hz(-1)) = 24.2 +/- 0.1.