Using empty-field `Quick Look' images from the first two epochs of the VLA
Sky Survey (VLASS) observations, centred on the positions of ∼3700
individually radio-non-detected active galactic nuclei (AGNs) at z≥4, we
performed image stacking analysis to examine the sub-mJy emission at 3 GHz.
We found characteristic monochromatic radio powers of Pchar=(2−13)×1024 W Hz−1, Pchar=2×1024−1.3×1025
W Hz−1, indicating that AGN-related radio emission is widespread in the
sample. The signal-to-noise ratios of the redshift-binned median stacked maps
are between 4−6, and we expect that with the inclusion of the yet to be
completed third-epoch VLASS observations, the detection limit defined as
signal-to-noise ratio SNR≥6 could be reached, and the redshift
dependence can be determined. To obtain information on the general spectral
properties of the faint radio emission in high-redshift AGNs, we confined the
sample to ∼3000 objects covered by both the VLASS and the Faint Images of
the Radio Sky at Twenty-centimeters (FIRST) survey. We found that the flux
densities from the median stacked maps show a characteristic spectral index of
α∗=−0.30±0.15, which is in agreement with the median spectral index
of the radio-detected z≥4 AGNs from our high-redshift AGN catalogue. The
three-band mid-infrared colour--colour diagram based on Wide-field Infrared
Survey Explorer observations provides further support regarding the AGN
contribution to the radio emission in the sub-mJy sample.Comment: 9 pages,7 figures, 3 tables. Accepted for publication in MNRA