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

Research output: Contribution to journalArticlepeer-review


We present the discovery and preliminary characterization of a gravitationally lensed quasar with a source redshift zs = 2.74 and image separation of 2.9 arcsec lensed by a foreground zl = 0.40 elliptical galaxy. Since optical observations of gravitationally lensed quasars showthe lens system as a 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 comprising 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 arcsec, enclosed mass Menc ~ 4 × 1011 M and a time delay of ~52 d. 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 (US)
Pages (from-to)4325-4334
Number of pages10
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
StatePublished - Mar 11 2017


  • Gravitational lensing: strong
  • Methods: observational
  • Methods: statistical
  • Quasars: general

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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