UNIONS Optical Identifications for VLASS Radio Sources in the Euclid Sky (UNVEIL) I. A Catalog of ~146,000 Radio Galaxies up to z~5

TL;DR

This paper presents a comprehensive catalog of approximately 146,000 radio galaxies identified through optical and radio surveys, extending the known population of radio-loud AGNs up to redshift ~5, and provides multi-frequency radio data for diverse spectral analyses.

Contribution

It introduces a large, multi-wavelength catalog of radio galaxies with redshift estimates, significantly increasing the known high-redshift RLAGN population and enabling detailed spectral studies.

Findings

Identified ~146,000 radio galaxies with optical counterparts up to z~5.

Found over 49,000 radio-loud AGNs with L_1.4GHz > 10^24 W/Hz.

Revealed diverse spectral shapes including steep, peaked, and upturned spectra.

Abstract

We present results of optical identifications for Very Large Array Sky Survey (VLASS) radio sources using the Ultraviolet Near Infrared Optical Northern Survey (UNIONS). A cross-match between UNIONS and VLASS Epoch 2 catalogs yields 146,212 radio galaxies down to $r=24.5$ mag over a wide area of $\sim4,200\ \mathrm{deg^2}$. We perform g-dropout selections and find $>200$ sources at $z\sim4$ optimistically. Of 63,019 sources with valid photo-$z$, 8,692 are at $z_\mathrm{photo}>1$, and 1,171 are at $z_\mathrm{photo}>2$. Based on spectral luminosities at 1.4 GHz using the valid photo-$z$, we identify $\sim49,000$ radio-loud AGNs (RLAGNs) with $\textit{L}_\mathrm{1.4GHz}>10^{24}$ W/Hz, and all radio galaxies at $z>1$ are RL. Adopting radio loudness instead, 138,266 out of 146,212 UNIONS-VLASS radio galaxies are RL. Thus, our catalog greatly increases the number counts of RLAGNs at $z>1$. We further cross-match the UNIONS-VLASS catalog with LOw-Frequency ARray Two-metre Sky Survey (LoTSS) at 144 MHz and Faint Images of the Radio Sky at Twenty-cm (FIRST) at 1.4 GHz, yielding 101,671 UNIONS-VLASS-LoTSS, 79,638 -FIRST, and 64,672 -LoTSS-FIRST sources, respectively. This multifrequency radio dataset reveals sources of various spectral shapes, including the steep spectrum of aged populations, the peaked spectrum of young populations, and the upturned spectrum potentially associated with transient sources. The UNIONS-VLASS radio galaxies will be covered by the Euclid wide survey, offering legacy values to benefit multi-faceted studies related to RLAGNs and beyond.