CURLING - III. Identifying Candidates of Wide-separation Gravitationally Lensed Quasars from the CatNorth Catalogue

Abstract

Wide-separation lensed quasars (WSLQs) are a rare subclass of strongly lensed quasars produced by massive galaxy clusters. They provide valuable probes of dark-matter halos and quasar host galaxies. However, only about ten WSLQ systems are currently known, which limits further studies. To enlarge the sample from wide-area surveys, we developed a catalog-based pipeline and applied it to the CatNorth database, a catalog of quasar candidates constructed from Gaia DR3. CatNorth contains 1,545,514 quasar candidates with about 90% purity and a Gaia G-band limiting magnitude of roughly 21. The pipeline has three stages. First, we identify groups with separations between 10 and 72 arcsec using a HEALPix grid with 25.6 arcsec spacing and a friends-of-friends search. We then filter by intra-group color and spectral similarity, reducing the 1,545,514 sources to 14,244 groups while retaining all known, discoverable WSLQs. Finally, a visual check, guided by image geometry and the presence of likely foreground lenses, yields the candidate list with quality labels. We identify 333 new WSLQ candidates with separations from 10 to 56.8 arcsec. Using available SDSS DR16 and DESI DR1 spectroscopy, we uncover two new candidate systems; the remaining 331 candidates lack sufficient spectra and are labeled as 45 grade A, 98 grade B, and 188 grade C. We also compile 29 confirmed dual quasars as a by-product. When feasible, we plan follow-up spectroscopy and deeper imaging to confirm WSLQs among these candidates and enable the related science.

Figures (12)

• Figure 1: Optical images of the four discoverable wide-separation lensed quasars. From left to right: SDSS J1004+4112, SDSS J1029+2623, SDSS J1326+4806, and GraL J165105.3$-$041725. Quasar images included in CatNorth are marked with white circles. The quasar images that belong to known WSLQ systems but are not included in CatNorth are marked as red circles we only plot the non-odd image). The images of the first three panels are from DESI Legacy Imaging Surveys DR9, and the rightmost panel is from Pan-STARRS1. (Orientation: north is up, south is down, west is left, and east is right; same hereafter.)
• Figure 2: Flowchart illustrating the methodology employed to select wide-separation lensed quasar candidates and dual quasar candidates from the CatNorth.
• Figure 3: Distribution of the angular separation between the mean sky position of each quasar group and the BCG of the nearest matched cluster (maximum allowed match radius $2^\prime$). Orange bars (right $y$-axis) show all matches from the QGC (pre-VI), while blue bars (left $y$-axis) show the corresponding results of visually inspected LQC (post-VI).
• Figure 4: DESI Legacy Imaging Surveys DR9 $grz$ composite images of the five Grade-A candidates for which at least one galaxy cluster is located within 30 arcsec of the quasar group centre. Each image spans 70 arcsec $\times$ 70 arcsec. White circles indicate the quasar candidate images, while red and blue circles mark the positions of the BCG matched in WEN_CAT and ZOU_CAT, respectively.
• Figure 5: DESI Legacy Imaging Surveys DR9 $grz$ composite images of the first fifteen Grade-A candidates for which no galaxy cluster is matched within 30 arcsec of the quasar group centre. Each panel spans 70 arcsec $\times$ 70 arcsec. White circles mark the positions of the quasar candidate images.