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 (1) , Emmanuel Bertin (1) , David Brooks , Elizabeth Buckley-Geer , Aurelio Carnero Rosell , Matias Carrasco Kind , Jorge Carretero , Carlos E. Cunha , Luiz N. da Costa , Shantanu 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

1 IAP - Institut d'Astrophysique de Paris

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 arcsec lensed by a foreground = 0.40 elliptical galaxy. Since optical observations of gravitationally lensed quasars show the 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 i_AB = 18.61 and i_AB = 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 θ_E ∼ 1.47 arcsec, enclosed mass M_enc ∼ 4 × 10^11 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.

Mots clés: quasars: general; methods: statistical; gravitational lensing: strong; methods: observational

Domaines: Astrophysique [astro-ph]

Fernanda Ostrovski, Richard G. Mcmahon, Andrew J. Connolly, Cameron A. Lemon, Matthew W. Auger, et al.. VDES J2325−5229 a z = 2.7 gravitationally lensed quasar discovered using morphology-independent supervised machine learning. Monthly Notices of the Royal Astronomical Society, 2017, 465 (4), pp.4325-4334. ⟨10.1093/mnras/stw2958⟩. ⟨hal-01554491⟩