A Wide and Deep Exploration of Radio-detected Active Galactic Nuclei with Subaru HSC (WERGS). XII. Final Optical Identification of VLASS Radio Sources from the Subaru/HSC-SSP Wide Survey Over 1200 deg$^2$

TL;DR

This paper presents a comprehensive optical identification catalog for VLASS radio sources over 1200 deg$^2$, leveraging deep Subaru HSC imaging to improve source matching and enable detailed multi-frequency radio analysis.

Contribution

The study provides the largest optically identified radio source catalog to date, with enhanced positional accuracy and sensitivity to faint, high-redshift, and obscured AGN hosts.

Findings

Catalog contains 22,773 sources with optical counterparts.

Improved positional accuracy with median 0.199 arcsec.

Enhanced sensitivity to optically faint and high-redshift hosts.

Abstract

We present a wide-area and deep optical identification catalog for radio sources based on the VLASS Epoch 2 catalog at 3 GHz. Optical counterparts are identified using the final-year internal processing of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) Wide layer (DR S23B), which provides deep imaging over ~1200 deg$^2$ in $grizy$ with $i$-band depth of $i_\mathrm{AB}\sim26$. Starting from a 1.0 arcsec nearest-neighbor match between VLASS and HSC, we construct a quality-controlled primary catalog (Clean VLASS-HSC) of 22,773 sources by requiring S/N$>5$ in at least one HSC band. We further provide ancillary nearest-neighbor associations to FIRST (1.4 GHz) and LoTSS DR3 (150 MHz) within 2.5 arcsec, resulting in 18,444 FIRST-matched sources, 16,167 LoTSS-matched sources, and a 14,206-source subset matched to both surveys. The catalog contains approximately six times more optically identified radio sources than the first WERGS optical-identification effort based on the early HSC-SSP S16B data and positional cross-matching with FIRST (Yamashita et al. 2018). The VLASS Epoch 2 resolution yields more precise optical associations (median 0.199 arcsec) and, together with uniform FIRST/LoTSS matches, enables robust multi-frequency radio SED constraints. Compared to UNIONS-based VLASS identifications (Zhong et al. 2025), the deeper HSC imaging improves sensitivity to optically faint and even morphologically resolved hosts at $z\gtrsim1$. Our catalog preferentially highlights host-dominated AGN candidates, potentially including a substantial fraction of obscured systems.