VDES J2325-5229 a z=2.7 gravitationally lensed quasar discovered using morphology independent supervised machine learning

Fernanda Ostrovski, Richard G. McMahon, Andrew J. Connolly, Cameron A. Lemon, Matthew W. Auger, Manda Banerji, Johnathan M. Hung, Sergey E. Koposov, Christopher E. Lidman, Sophie L. Reed, Sahar Allam, Aurélien Benoit-Lévy, Emmanuel Bertin, David Brooks, Elizabeth Buckley-Geer, Aurelio Carnero Rosell, Matias Carrasco Kind, Jorge Carretero, Carlos E. Cunha, Luiz N. da CostaShantanu Desai, H. Thomas Diehl, Jörg P. Dietrich, August E. Evrard, David A. Finley, Brenna Flaugher, Pablo Fosalba, Josh Frieman, David W. Gerdes, Daniel A. Goldstein, Daniel Gruen, Robert A. Gruendl, Gaston Gutierrez, Klaus Honscheid, David J. James, Kyler Kuehn, Nikolay Kuropatkin, Marcos Lima, Huan Lin, Marcio A. G. Maia, Jennifer L. Marshall, Paul Martini, Peter Melchior, Ramon Miquel, Ricardo Ogando, Andrés Plazas Malagón, Kevin Reil, Kathy Romer, Eusebio Sanchez, Basilio Santiago, Vic Scarpine, Ignacio Sevilla-Noarbe, Marcelle Soares-Santos, Flavia Sobreira, Eric Suchyta, Gregory Tarle, Daniel Thomas, Douglas L. Tucker, Alistair R. Walker

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We present the discovery and preliminary characterization of a gravitationally lensed quasar with a source redshift z= 2.74 and image separation of 2.9" lensed by a foreground z= 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 iAB = 18.61 and iAB = 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 = 2.739 ± 0.003 and a foreground early type galaxy with z = 0.400 ± 0.002. We model the system as a single isothermal ellipsoid and find the Einstein radius θE  ∼ 1.47", enclosed mass Menc  ∼ 4 x 1011MΘ and a time delay of ∼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.
Original languageEnglish
Pages (from-to)4325-4334
Number of pages10
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
Publication statusPublished - 17 Nov 2016


  • astro-ph.GA
  • RCUK
  • STFC


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