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 work constructs the UNIONS-VLASS Radio Galaxy Catalog by cross-matching deep optical data from UNIONS with the VLASS radio survey, producing a large sample of ~146{,}000 radio galaxies up to $z\sim5$ over ~4,200 deg$^2$. The catalog integrates multi-survey identifications (FIRST, LoTSS) and provides extensive metadata, including photometric/spectroscopic redshifts, radio spectral indices, luminosities, and morphology classifications. The authors quantify contamination/completeness, analyze optical/radio number counts, and identify radio-loud AGNs via both radio loudness and luminosity thresholds, revealing strong RLAGN dominance at $z>1$ and highlighting HzRG候補 via dropout techniques. The dataset, especially when combined with Euclid, offers a powerful legacy resource for RLAGN studies, jet–host co-evolution, and high-redshift radio galaxy searches, with clear paths for future expansion as VLASS and UNIONS continue to grow.

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.

Figures (22)

• Figure 1: Flowchart of the cross-match processes and sample selections (see § \ref{['sec:xmatched sources']} for details). In cyan boxes that describe the sample sizes, we supplement the sections where the corresponding sample is described and discussed.
• Figure 2: The angular separation-$\mathtt{NN\_dist}$ plane for UNIONS-VLASS sources cross-matched with a search radius of $6"$. The vertical lines indicate the $\mathtt{NN\_dist}$ cut of $30"$ and the horizontal line represents search radius of $1.5"$ to perform the cross-match (see § \ref{['subsec:completeness']} and \ref{['sec: optical-radio correspondence']}).
• Figure 3: Leftmost: Tile-wise number densities ${n}_\mathrm{den}$ of galaxies per $\mathrm{arcmin^2}$ for UNIONS tiles in the North Galactic Cap. The violet, blue, and orange lines indicate average, median, and maximal ${n}_\mathrm{den}$, respectively. Middle: completeness and cleanness as a function of the search radius ${r}_\mathrm{s}$. The vertical dashed lines mark where the cleanness approximately intersects the completeness, indicating a trade-off between the totality of the catalog and the exclusion of the contamination. Rightmost: differential distribution of separation between UNIONS optical and VLASS radio sources including radio sources that are associated with multiple optical counterparts.
• Figure 4: Astrometric offsets in R.A. ($\Delta\alpha$) and DEC. ($\Delta\delta$) for 146,212 "clean" UNIONS-VLASS sources (blue) associated via $\sqrt{\Delta\alpha^2+\Delta\delta^2}\leq1.5\arcsec$ and subsample of 140,065 source adopting positional cross-match (orange; see texts for details). Mean values of the offsets are embedded in the bracket and $\sigma$ represents the standard deviation. The typical astrometric offset of VLASS Epoch 1 is $\sim0.5\arcsec$ at $\mathrm{DEC.>-20\ deg^2}$.
• Figure 5: The sky coverages of UNIONS (black), VLASS (cyan), FIRST (green), and LoTSS (orange).