Radio-loud fraction of z>6 quasars

Abstract

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 HI 21cm absorption studies. However, the lack of deep radio observations of $z>6$ quasars leaves the population poorly constrained, and suitable candidates for an HI 21cm 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 $6.0\le z<7.6$. We detect the radio continuum emission of the $z=6.1$ quasar J1034-1425, with a 1.6 GHz flux density of $170\pm 36\mu$Jy. This quasar is radio-quiet with radio-loudness, $R\equiv f_{5\text{ GHz}}/f_{\nu ,\text{4400 A}}=2.4\pm 0.5$. 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 $3.8_{-2.4}^{+6.2}\%$, 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 $13.8\pm 3.9\mu$Jy and luminosity of $\log L_{5\mathrm{GHz}}/({\mathrm{W}\mathrm{Hz}}^{-1})=24.2\pm 0.1$.

12 pages, 8 figures, 3 tables, published in MNRAS

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