A Wide and Deep Exploration of Radio-detected Active Galactic Nuclei with Subaru HSC (WERGS). XIII. High-Redshift Radio Quasar candidates beyond Ultra-Steep Spectrum Selection: Dropout selection from HSC--VLASS over $\sim$1200 deg${}^2$

We report the results of $g-$, $r-$, and $i-$dropout selections based on optical identifications of Very Large Array Sky Survey (VLASS) radio sources using the Hyper Suprime-Cam Subaru Strategic Program survey (HSC–SSP). By positional crossmatching within $1^{\mathrm{\prime }\mathrm{\prime }}.5$ between the VLASS Epoch~2 catalog and the HSC–SSP Wide-layer catalog ($i\lesssim 26$), we obtain $\sim$400 high-redshift radio AGN candidates at $z\gtrsim 4$ over a $\approx 1200{\mathrm{d}\mathrm{e}\mathrm{g}}^2$ survey footprint, extending optimistically to $z\sim 7$. Optical magnitudes cluster at $i_{\mathrm{A}\mathrm{B}}\simeq 24$–26, indicating that these sources are largely inaccessible to shallower surveys such as SDSS. By further cross-matching the HSC–VLASS dropout catalog with VLA Faint Images of the Radio Sky at Twenty-centimeters (FIRST) at 1.4~GHz, the LOFAR Two-metre Sky Survey (LoTSS) at 144~MHz, and the TIFR GMRT Sky Survey (TGSS) at 150~MHz, the majority of the high-$z$ candidates show flat to moderately steep radio spectra ($-1\lesssim \alpha \lesssim 0$, with $f_{\nu }\propto {\nu }^{\alpha }$), and some also exhibit turnover radio spectra, demonstrating that conventional ultra-steep-spectrum (USS; $\alpha <-1.3$) selection would miss most of the population selected in this study. Building on this, we perform SED fitting and obtain AGN luminosities, which show a clustering at typical bolometric luminosities of $\log (L_{\mathrm{b}\mathrm{o}\mathrm{l}}\mathrm{/}\mathrm{e}\mathrm{r}\mathrm{g}{\mathrm{s}}^{-1})\sim 46$–47. We also examine the comoving number density distribution of our samples and find a sharp decline around the $i$-dropout regime ($z\sim 6$), suggesting the possible disappearance of luminous radio AGNs toward the epoch of reionization.