Gravitationally Lensed Quasars in Gaia: I. Resolving Small-Separation Lenses

TL;DR

This study leverages Gaia's high resolution to identify and analyze small-separation gravitationally lensed quasars, discovering new candidates and demonstrating a method to detect such systems using Gaia data alone.

Contribution

The paper introduces a novel approach using Gaia data to detect small-separation lensed quasars, including new candidate discoveries and a method applicable to future surveys.

Findings

Detected 8 known lensed quasars with multiple Gaia components

Identified 4 new candidate lensed quasars with sub-arcsecond separation

Proposed a method to find approximately 1400 lensed quasars with separations above 0.5 arcseconds

Abstract

Gaia's exceptional resolution (FWHM $\sim$ 0.1$^{\prime\prime}$) allows identification and cataloguing of the multiple images of gravitationally lensed quasars. We investigate a sample of 49 known lensed quasars in the SDSS footprint, with image separations less than 2$^{\prime\prime}$, and find that 8 are detected with multiple components in the first Gaia data release. In the case of the 41 single Gaia detections, we generally are able to distinguish these lensed quasars from single quasars when comparing Gaia flux and position measurements to those of Pan-STARRS and SDSS. This is because the multiple images of these lensed quasars are typically blended in ground-based imaging and therefore the total flux and a flux-weighted centroid are measured, which can differ significantly from the fluxes and centroids of the individual components detected by Gaia. We compare the fluxes through an empirical fit of Pan-STARRS griz photometry to the wide optical Gaia bandpass values using a sample of isolated quasars. The positional offsets are calculated from a recalibrated astrometric SDSS catalogue. Applying flux and centroid difference criteria to spectroscopically confirmed quasars, we discover 4 new sub-arcsecond-separation lensed quasar candidates which have two distinct components of similar colour in archival CFHT or HSC data. Our method based on single Gaia detections can be used to identify the $\sim$ 1400 lensed quasars with image separation above 0.5$^{\prime\prime}$, expected to have only one image bright enough to be detected by Gaia.