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$

TL;DR

This study identifies approximately 400 high-redshift radio AGN candidates beyond traditional ultra-steep-spectrum selection using dropout techniques on combined optical and radio surveys over 1200 deg$^2$, revealing a population missed by previous methods.

Contribution

It introduces a dropout-based selection method for high-redshift radio AGNs that captures sources with diverse radio spectra, extending beyond ultra-steep-spectrum criteria.

Findings

Most high-$z$ candidates have flat to moderate radio spectra.

The population shows bolometric luminosities around 10^{46}–10^{47} erg s$^{-1}$.

Number density declines sharply at $z \,\sim 6$, indicating fewer luminous AGNs near reionization.

Abstract

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 $1''.5$ 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 $\approx1200~\mathrm{deg}^2$ survey footprint, extending optimistically to $z \sim 7$. Optical magnitudes cluster at $i_\mathrm{AB} \simeq 24$--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 $\log(L_{\rm bol}/{\rm erg~s^{-1}})\sim46$--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.