Issue |
A&A
Volume 659, March 2022
|
|
---|---|---|
Article Number | A140 | |
Number of page(s) | 14 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/202142389 | |
Published online | 18 March 2022 |
Discovery of strongly lensed quasars in the Ultraviolet Near Infrared Optical Northern Survey (UNIONS)
1
Institute of Physics, Laboratory of Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
e-mail: hung-hsu.chan@epfl.ch
2
Institut d’Astrophysique de Paris, UMR7095 CNRS & Sorbonne Université, 98bis Bd Arago, 75014 Paris, France
3
Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB30HA, UK
4
AIM, CEA, CNRS, Université Paris-Saclay, Université de Paris, 91191 Gif-sur-Yvette, France
5
NRC Herzberg Astronomy & Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
6
Waterloo Centre for Astrophysics, University of Waterloo, 200, University Ave W, Waterloo, ON N2L 3G1, Canada
7
Department of Physics and Astronomy, University of Waterloo, Waterloo, ON N2L 3G1, Canada
8
Perimeter Institute for Theoretical Physics, 31 Caroline St N, Waterloo, ON N2L 2Y5, Canada
Received:
7
October
2021
Accepted:
13
November
2021
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.
Key words: gravitational lensing: strong / quasars: general
© ESO 2022
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