Searching for strong lensing by late-type galaxies in UNIONS

¹ Institute of Physics, Laboratory of Astrophysics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny 1290 Versoix, Switzerland
² SCITAS, Ecole Polytechnique Fédérale de Lausanne (EPFL) 1015 Lausanne, Switzerland
³ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB) Martí i Franquès 1 08028 Barcelona, Spain
⁴ Institució Catalana de Recerca i Estudis Avançats (ICREA) Passeig de Lluís Companys 23 08010 Barcelona, Spain
⁵ Laboratoire d’Astrophysique de Marseille, UMR7326, Aix-Marseille Université, CNRS, CNES 13013 Marseille, France
⁶ Institut d’Astrophysique de Paris, UMR 7095, CNRS, and Sorbonne Université 98 bis boulevard Arago 75014 Paris, France
⁷ Oskar Klein Centre, Department of Physics, Stockholm University SE-106 91 Stockholm, Sweden
⁸ University of Applied Sciences and Arts of Northwestern Switzerland, School of Engineering 5210 Windisch, Switzerland
⁹ Institute of Cosmology and Gravitation, University of Portsmouth Burnaby Rd Portsmouth PO1 3FX, UK
¹⁰ Department of Physics and Astronomy, University of British Columbia Vancouver BC V6T 1Z1, Canada
¹¹ Canadian Astronomy Data Centre, Herzberg Astronomy and Astrophysics, National Research Council 5071 West Saanich Rd Victoria BC V9E 2E7, Canada
¹² LIRA, Observatoire de Paris, Université PSL, CNRS Meudon, France
¹³ Department of Physics and Astronomy, University of Waterloo Waterloo ON N2L 3G1, Canada
¹⁴ Waterloo Centre for Astrophysics Waterloo ON N2L 3G1, Canada
¹⁵ Perimeter Institute for Theoretical Physics 31 Caroline St. N. Waterloo ON N2L 2Y5, Canada
¹⁶ Institute for Astronomy, University of Hawaii 2680 Woodlawn Drive Honolulu HI 96822, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 15 March 2025
Accepted: 3 November 2025

Abstract

Recent wide-field galaxy surveys have led to an explosion in the number of galaxy-scale strong gravitational lens candidates. However, the vast majority of them feature massive luminous red galaxies as the main deflectors, with late-type galaxies being vastly under-represented. This work presents a dedicated search for lensing by edge-on late-type galaxies in the Ultraviolet Near Infrared Optical Northern Survey (UNIONS). The search covers 3600 deg² of r-band observations taken from the Canada-France-Hawaii Telescope. We considered all sources with magnitudes in the range 17 < r < 20.5, without any colour pre-selection, yielding a parent sample of seven million sources. We characterised our parent sample via the visual inspection of 120 000 sources selected at random. From it, we estimate, with a 68% confidence interval, that 1 in every 30 000 sources is an edge-on lens candidate, with at least eight high-quality candidates in the parent sample. This corresponds to one candidate per 17 000 edge-on late-type galaxies. Our search relied on a convolutional neural network (CNN) to select a reduced sample of candidates, which we followed with a visual inspection to curate the final sample. The CNN was trained from scratch using simulated r-band observations of edge-on lenses, and real observations of non-lenses. We found 61 good edge-on lens candidates using the CNN. Moreover, combining the CNN candidates with those found serendipitously and those identified while characterising the parent sample, we discovered 4 grade A, 20 grade B, and 58 grade C edge-on lens candidates, effectively doubling the known sample of these systems. We also discovered 16 grade A, 16 grade B, and 18 grade C lens candidates of other types. Finally, based on the characterisation of the parent sample, we estimate that our search found around 60% of the bright grade A and B edge-on lens candidates within the parent sample.