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Using empty-field `Quick Look' images from the first two epochs of the VLASky Survey (VLASS) observations, centred on the positions of $\sim3700$individually radio-non-detected active galactic nuclei (AGNs) at $z\ge4$, weperformed image stacking analysis to examine the sub-mJy emission at $3$ GHz.We found characteristic monochromatic radio powers of $P_\mathrm{char}=(2-13)\times 10^{24}$ W Hz$^{-1}$, $P_\mathrm{char}=2\times10^{24}-1.3\times10^{25}$W Hz$^{-1}$, indicating that AGN-related radio emission is widespread in thesample. The signal-to-noise ratios of the redshift-binned median stacked mapsare between $4-6$, and we expect that with the inclusion of the yet to becompleted third-epoch VLASS observations, the detection limit defined assignal-to-noise ratio $\mathrm{SNR}\ge6$ could be reached, and the redshiftdependence can be determined. To obtain information on the general spectralproperties of the faint radio emission in high-redshift AGNs, we confined thesample to $\sim3000$ objects covered by both the VLASS and the Faint Images ofthe Radio Sky at Twenty-centimeters (FIRST) survey. We found that the fluxdensities from the median stacked maps show a characteristic spectral index of$\alpha^*=-0.30\pm0.15$, which is in agreement with the median spectral indexof the radio-detected $z\ge4$ AGNs from our high-redshift AGN catalogue. Thethree-band mid-infrared colour--colour diagram based on Wide-field InfraredSurvey Explorer observations provides further support regarding the AGNcontribution to the radio emission in the sub-mJy sample.

Sub-mJy radio emission from high-redshift active galactic nuclei in the footprint of the VLA Sky Survey