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

Abstract We present the results of optical identifications for Very Large Array Sky Survey (VLASS) radio sources employing the Ultraviolet Near Infrared Optical Northern Survey (UNIONS). A crossmatch between UNIONS and VLASS Epoch 2 catalogs yields 146,212 radio galaxies over a wide area of ∼4200 deg 2 in the Northern Hemisphere. We perform g- dropout selections and find ≳200 sources at z ∼ 4 optimistically. Of 63,019 sources with valid photometric redshifts, 8692 are at z photo ≥ 1 and 1171 are at z photo ≥ 2. Based on spectral luminosities at 1.4 and 3 GHz using the valid UNIONS photo- z , we identify ∼49,000 radio-loud active galactic nuclei (RLAGNs) with L 1.4 GHz > 10 24 W Hz −1 , and all radio galaxies at z ≥ 1 are radio-loud (RL). Adopting radio-loudness instead, 138,266 out of 146,212 UNIONS-VLASS radio galaxies are RL. By virtue of the wide area and medium depth provided by UNIONS, our catalog greatly increases the number counts of RLAGNs at z > 1. We further crossmatch the UNIONS-VLASS catalog with the LOFAR Two-metre Sky Survey (LoTSS) at 144 MHz and VLA Faint Images of the Radio Sky at Twenty-cm (FIRST) at 1.4 GHz, yielding 101,671 UNIONS-VLASS-LoTSS, 79,638 UNIONS-VLASS-FIRST, and 64,672 UNIONS-VLASS-LoTSS-FIRST sources, respectively. This multifrequency radio data set reveals sources of various spectral shapes, including the steep spectrum of aged populations, the peaked spectrum of young populations, and the upturned spectrum that might be associated with transient sources. The UNIONS-VLASS radio galaxy candidates will be covered by the Euclid wide survey, bringing about legacy values to benefit multifaceted studies related to radio galaxies and beyond.