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$^2$

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.

Figures (2)

• Figure 1: Sky footprints of the surveys used in this work, shown in equatorial coordinates. Dots indicate the sky positions of cataloged sources in each survey: VLASS Epoch 2 (cyan), FIRST (purple), LoTSS DR3 (orange), and the HSC-SSP Wide-layer matched sample (black). The sources detected both in VLASS Epoch 2 and LoTSS DR3 are shown with green because of the combined color of the both. The black outline highlights the approximate footprint of the HSC-SSP Wide layer.
• Figure 2: Flowchart summarizing the construction of our multi-wavelength samples. Starting from the VLASS Epoch 2 SE component catalog and the HSC-SSP DR S23B Wide-layer photometric catalog, we apply a series of quality cuts to define the Clean VLASS and Clean HSC-SSP samples. We then perform a $1\hbox{$.\!\!^{\prime\prime}$}0$ positional match between Clean VLASS and Clean HSC-SSP to construct the Clean VLASS--HSC catalog ($N=22{,}773$; red-outlined box), which serves as the parent sample for subsequent cross-matching. From this parent catalog, we cross-match to the compact FIRST source catalog (selected with SIDEPROB$<0.05$) and to the LoTSS DR3 catalog within $2\hbox{$.\!\!^{\prime\prime}$}5$, producing the Clean VLASS--HSC--FIRST ($N=18{,}444$), Clean VLASS--HSC--LoTSS ($N=16{,}167$), and the intersection sample with counterparts in both FIRST and LoTSS, Clean VLASS--HSC--FIRST--LoTSS ($N=14{,}206$). Blue-outlined boxes denote auxiliary subsamples with reliable photometric redshifts, selected by $\chi^2_\nu<3$ and $\sigma_z<0.1(1+z)$, yielding $N=15{,}897$ (Clean VLASS--HSC with reliable photo-$z$), $N=12{,}649$ (Clean VLASS--HSC--FIRST with reliable photo-$z$), $N=11{,}077$ (Clean VLASS--HSC--LoTSS), and $N=9,627$ (Clean VLASS--HSC--FIRST--LoTSS with reliable photo-$z$).