A Wide and Deep Exploration of Radio-detected Active Galactic Nuclei with Subaru HSC (WERGS). XIII. High-z Radio Quasar Selection from HSC--VLASS over $\sim 1200$ deg${}^2$

Abstract

We report the results of $g-$, $r-$, $i-$, and $z-$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 $1{.}^{\prime \prime }5$ 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 $\approx 1200{\mathrm{deg}}^2$ survey footprint, extending optimistically to $z\sim 7$. Their $i$-band AB magnitudes range from $i_{\mathrm{AB}}\simeq 24$--26, placing most below the SDSS detection limit. By further crossmatching 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, we obtain multi-frequency radio measurements for the majority of the sample. Approximately 85\% of the sources have three radio detections. The majority of the high-$z$ candidates show flat to moderately steep radio spectra ($-1\lesssim \alpha \lesssim 0$, with $f_{\nu }\propto {\nu }^{\alpha }$), while some also exhibit turnover radio spectra, demonstrating a broad diversity of radio spectral properties among the dropout-selected candidates. SED fitting yields typical AGN bolometric luminosities of $\log (L_{\mathrm{bol}}/\mathrm{erg}{\mathrm{s}}^{-1})\sim 46.5$--47.5. We also find that the comoving number density declines from the $g$-dropout to the $z$-dropout regimes, with substantial decreases toward higher redshift, suggesting fewer luminous radio AGN candidates toward reionization.

Accepted for publication in ApJS