UNIONS: The Ultraviolet Near-infrared Optical Northern Survey

The Ultraviolet Near-Infrared Optical Northern Survey (UNIONS) is a “collaboration of collaborations” that is using the Canada–France–Hawai’i Telescope, the Pan-STARRS telescopes, and the Subaru Observatory to obtain ugriz images of a core survey region of 6250 deg2 of the northern sky. The 10σ point source depth of the data, as measured within a 2″ diameter aperture, are [u, g, r, i, z] = [23.7, 24.5, 24.2, 23.8, 23.3] in AB magnitudes. UNIONS is addressing some of the most fundamental questions in astronomy, including the properties of dark matter, the growth of structure in the Universe from the very smallest galaxies to large-scale structure, and the assembly of the Milky Way. It is set to become a major ground-based legacy survey for the northern hemisphere for the next decade, and it provides an essential northern complement to the static-sky science of the Vera C. Rubin Observatory’s Legacy Survey of Space and Time. UNIONS supports the core science mission of the Euclid space mission by providing the data necessary in the northern hemisphere for the calibration of the wavelength dependence of the Euclid point-spread function and derivation of photometric redshifts in the North Galactic Cap. This region contains the highest quality sky for Euclid, with low backgrounds from the zodiacal light, stellar density, extinction, and emission from Galactic cirrus. Here, we describe the UNIONS survey components, science goals, data products, and the current status of the overall program.