62 million galaxies mapped for dark matter across the northern sky

F. Hervas-Peters, S. Guerrini, M. Kilbinger, L. Baumont, A. Guinot, C. Daley, C. Bonini, A. Wittje, C. Murray, L. W. K. Goh, A. Paradis, A. Tersenov, M. J. Hudson, L. Van Waerbeke, H. Hildebrandt, S. Fabbro, J. -C. Cuillandre, A. W. McConnachie

The UNIONS collaboration has constructed a weak gravitational lensing catalogue from r-band imaging taken with MegaCam on the Canada-France Hawaii Telescope, covering roughly 3,500 square degrees of the Northern Hemisphere at a median seeing of 0.7 arcseconds. Careful PSF modelling, masking, and ellipticity calibration yield 62 million galaxies at an effective source density of 4.96 per square arcminute. The catalogue is validated against PSF leakage, bright-star proximity, and other systematic tracers. It is the first of five companion papers, with the others delivering shear calibration via image simulations, B-mode null tests, and full cosmic-shear cosmological results.