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Discovering strongly lensed quasar candidates with catalogue-based methods from DESI Legacy Surveys

He, Zizhao; Li, Nan; Cao, Xiaoyue; Li, Rui; Zou, Hu; Dye, Simon

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Zizhao He

Nan Li

Xiaoyue Cao

Rui Li

Hu Zou

Associate Professor


Context. The Hubble tension, revealed by a ~5σ discrepancy between measurements of the Hubble-Lemaitre constant among observations of the early and local Universe, is one of the most significant problems in modern cosmology. In order to better understand the origin of this mismatch, independent techniques to measure H0, such as strong lensing time delays, are required. Notably, the sample size of such systems is key to minimising the statistical uncertainties and cosmic variance, which can be improved by exploring the datasets of large-scale sky surveys such as Dark Energy Spectroscopic Instrument (DESI).

Aims. We identify possible strong lensing time-delay systems within DESI by selecting candidate multiply imaged lensed quasars from a catalogue of 24 440 816 candidate QSOs contained in the ninth data release of the DESI Legacy Imaging Surveys (DESI-LS).

Methods. Using a friend-of-friends-like algorithm on spatial co-ordinates, our method generates an initial list of compact quasar groups. This list is subsequently filtered using a measure of the similarity of colours among a group’s members and the likelihood that they are quasars. A visual inspection finally selects candidate strong lensing systems based on the spatial configuration of the group members.

Results. We identified 620 new candidate multiply imaged lensed quasars (101 grade-A, 214 grade-B, 305 grade-C). This number excludes 53 known spectroscopically confirmed systems and existing candidate systems identified in other similar catalogues. When available, these new candidates will be further checked by combining the spectroscopic and photometric data from DESI.

Journal Article Type Article
Acceptance Date Jan 25, 2023
Online Publication Date Apr 10, 2023
Publication Date 2023-04
Deposit Date Apr 19, 2023
Publicly Available Date Apr 19, 2023
Journal Astronomy and Astrophysics
Electronic ISSN 1432-0746
Publisher EDP Sciences
Peer Reviewed Peer Reviewed
Volume 672
Article Number A123
Keywords gravitational lensing: strong / quasars: general / methods: data analysis / catalogs / surveys
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