Kavli Affiliate: Kevin Reil
| Summary:
We present the discovery and preliminary characterization of a gravitationally lensed quasar with a source redshift $z_s=2.74$ and image separation of $2.9"$ lensed by a foreground $z_l=0.40$ elliptical galaxy. Since the images of gravitationally lensed quasars are the superposition of multiple point sources and a foreground lensing galaxy, we have developed a morphology independent multi-wavelength approach to the photometric selection of lensed quasar candidates based on Gaussian Mixture Models (GMM) supervised machine learning. Using this technique and $gi$ multicolour photometric observations from the Dark Energy Survey (DES), near IR $JK$ photometry from the VISTA Hemisphere Survey (VHS) and WISE mid IR photometry, we have identified a candidate system with two catalogue components with $i_AB=18.61$ and $i_AB=20.44$ comprised of an elliptical galaxy and two blue point sources. Spectroscopic follow-up with NTT and the use of an archival AAT spectrum show that the point sources can be identified as a lensed quasar with an emission line redshift of $z=2.739pm0.003$ and a foreground early type galaxy with $z=0.400pm0.002$. We model the system as a single isothermal ellipsoid and find the Einstein radius $θ_E sim 1.47"$, enclosed mass $M_enc sim 4 times 10^11$M$_odot$ and a time delay of $sim$52 days. The relatively wide separation, month scale time delay duration and high redshift make this an ideal system for constraining the expansion rate beyond a redshift of 1.
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