New lensed quasars from the MUSCLES survey

TL;DR

This paper reports the discovery of new gravitationally lensed quasars from the MUSCLES survey, refining selection methods and expanding the known sample to aid cosmological and astrophysical studies.

Contribution

It introduces an improved color-separation diagnostic for identifying lensed quasars and reports the discovery of 4 new lenses, enhancing the existing catalog from SDSS and UKIDSS data.

Findings

Discovered 4 new lensed quasars, doubling previous numbers.

Refined selection method using wavelength-dependent color separation.

Estimated that current surveys find up to 70% of the total lensed quasar population.

Abstract

Gravitational lens systems containing lensed quasars are important as cosmological probes, as diagnostics of structural properties of the lensing galaxies and as tools to study the quasars themselves. The largest lensed quasar sample is the SDSS Quasar Lens Search, drawn from the Sloan Digital Sky Survey (SDSS). We are attempting to extend this survey using observations of lens candidates selected from a combination of the quasar sample from the SDSS and the UKIRT Infrared Deep Sky Survey (UKIDSS). This adds somewhat higher image quality together with a wider range of wavelength for the selection process. In previous pilot surveys we observed 5 objects, finding 2 lenses; here we present further observations of 20 objects in which we find 4 lenses, of which 2 are independently discovered in SQLS (in preparation). Following earlier work on the combination of these two surveys, we have refined our method and find that use of a colour-separation diagnostic, where we select for separations between components which appear to decrease in wavelength, is an efficient method to find lensed quasars and may be useful in ongoing and future large-scale strong lensing surveys with instruments such as Pan-STARRS and LSST. The new lenses have mostly high flux ratios, with faint secondaries buried in the lensing galaxy and typically 6-10 times less bright than the primary. Our survey brings the total number of lenses discovered in the SDSS quasar sample to 46, plus 13 lenses already known. This is likely to be up to 60-70% of the total number of lensed quasars; we briefly discuss strategies by which the rest might be found.