We carry out a series of deep Karl G. Jansky Very Large Array (VLA) S-band observations of a sample of 21 quasars at z ∼ 6. The new observations expand the searches of radio continuum emission to the optically faint quasar population at the highest redshift with rest-frame 4400 Å luminosities down to 3 × 10 11 L ⊙ . We report the detections of two new radio-loud quasars: CFHQS J2242+0334 (hereafter J2242+0334) at z = 5.88 and CFHQS J0227−0605 (hereafter J0227−0605) at z = 6.20, detected with 3 GHz flux densities of 87.0 ± 6.3 μ Jy and 55.4 ± 6.7 μ Jy , respectively. Their radio loudnesses are estimated to be 54.9 ± 4.7 and 16.5 ± 3.2, respectively. To better constrain the radio-loud fraction (RLF), we combine the new measurements with the archival VLA L-band data as well as available data from the literature, considering the upper limits for non-detections and possible selection effects. The final derived RLF is 9.4 ± 5.7% for the optically selected quasars at z ∼ 6. We also compare the RLF to that of the quasar samples at low redshift and check the RLF in different quasar luminosity bins. The RLF for the optically faint objects is still poorly constrained due to the limited sample size. Our results show no evidence of significant quasar RLF evolution with redshift. There is also no clear trend of RLF evolution with quasar UV/optical luminosity due to the limited sample size of optically faint objects with deep radio observations.

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 α = −0.52 ± 0.18 (S ∝ ν α ). The radio source is unresolved in all VLA bands with an upper limit to the size of 0.″2 (i.e., ∼1.1 kpc) at 10 GHz. We find variability in the flux density (increase by ∼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-kiloparsec area, which is diffuse and resolved out in the VLBA observations. The diffuse emission is 4 times brighter than what 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.

High-resolution imaging is crucial for exploring the origin and mechanism of radio emission in quasars, especially at high redshifts. We present 1.5 GHz Very Long Baseline Array (VLBA) images of the radio continuum emission from the radio-intermediate quasar (RIQ) J2242+0334 at z = 5.9. This object was previously detected at both 1.5 GHz and 3 GHz with the Karl G. Jansky Very Large Array (VLA) as a point source. However, there is no clear detection in the VLBA images at both the full resolution of 10.7 mas × 4.5 mas (61.7 pc × 26.0 pc) and a tapered resolution of 26 mas × 21 mas (150 pc × 121 pc). This suggests that the radio emission from the quasar is diffuse on milliarcsecond scales with surface brightness fainter than the 3σ detection limit of 40.5 μJy beam−1 in the full-resolution image. The radio emission in the RIQ J2242+0334 is likely to be wind-like (i.e., diffuse) rather than in the form of collimated jets. This is different from the previous radio detections of the most luminous quasars at z ∼ 6 that are usually dominated by compact, high brightness temperature radio sources. Meanwhile, compared with RIQs at low redshifts, the case of J2242+0334 suggests that not all RIQs are beamed radio-quiet quasars. This optically faint RIQ provides an important and unique example to investigate the radio activity in the less powerful active galactic nuclei at the earliest cosmic epoch.