Discovery of strongly lensed quasars in the Ultraviolet Near Infrared Optical Northern Survey (UNIONS)

Abstract

We report the discovery of five new doubly imaged lensed quasars from the first 2500 square degrees of the ongoing Canada-France Imaging Survey (CFIS), which is a component of the Ultraviolet Near Infrared Optical Northern Survey. The systems are preselected in the initial catalogues of either Gaia pairs or MILLIQUAS quasars. We then take advantage of the deep, 0.6″median-seeing r-band imaging of CFIS to confirm the presence of multiple point sources with similar colour of u − r via convolution of the Laplacian of the point spread function. Requiring point sources of similar colour and with flux ratios of less than 2.5 mag in r-band, we reduce the number of candidates from 256 314 to 7815. After visual inspection, we obtain 30 high-grade candidates, and prioritise a spectroscopic follow-up analysis for those showing signs of a lensing galaxy upon subtraction of the point sources. We obtain long-slit spectra for 18 candidates with ALFOSC on the 2.56-m Nordic Optical Telescope, confirming five new doubly lensed quasars with 1.21 < z < 3.36 and angular separations from 0.8″ to 2.5″. One additional system is a probable lensed quasar based on the CFIS imaging and existing SDSS spectrum. We further classify six objects as nearly identical quasars, that is, possible lenses but without the detection of a lensing galaxy. Given our recovery rate (83%) of existing optically bright lenses within the CFIS footprint, we expect that a similar strategy, coupled with u − r colour-selection from CFIS alone, will provide an efficient and complete discovery of small-separation lensed quasars of source redshifts below z = 2.7 within the CFIS r-band magnitude limit of 24.1 mag.