J1721+8842: The first Einstein zigzag lens

¹ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
² Institute of Physics, Laboratory of Astrophysics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
³ Kavli Institute for Particle Astrophysics and Cosmology and Department of Physics, Stanford University, Stanford, CA 94305, USA
⁴ Institute for Particle Physics and Astrophysics, ETH Zurich, Wolfgang-Pauli-Strasse 27, CH-8093 Zurich, Switzerland
⁵ Oskar Klein Centre, Department of Physics, Stockholm University, SE-106 91 Stockholm, Sweden
⁶ UCLA Physics & Astronomy, 475 Portola Plaza, Los Angeles, CA 90095-1547, USA
⁷ Institut de Ciències del Cosmos, Universitat de Barcelona, Martí i Franquès, 1, E-08028 Barcelona, Spain
⁸ ICREA, Pg. Lluís Companys 23, Barcelona E-08010, Spain
⁹ Department of Astronomy & Astrophysics, University of Chicago, Chicago, IL 60637, USA
¹⁰ Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA
¹¹ Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794, USA
¹² Research Center for the Early Universe, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
¹³ DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 155A, DK-2200 Copenhagen N, Denmark
¹⁴ STFC Hartree Centre, Sci-Tech Daresbury, Keckwick Lane, Daresbury, Warrington WA4 4AD, UK
¹⁵ Universidad Andres Bellos, Fernández Concha 700, Av. Las Condes alt. 13.350, Las Condes, Santiago, Chile
¹⁶ Technical University of Munich, TUM School of Natural Sciences, Physics Department, James-Franck-Straße 1, 85748 Garching, Germany
¹⁷ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild Straße 1, 85748 Garching, Germany
¹⁸ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, I-20133 Milano, Italy
¹⁹ INAF – IASF Milano, Via A. Corti 12, I-20133 Milano, Italy
²⁰ STAR Institute, University of Liège, Quartier Agora, Allée du Six Août 19c, 4000 Liège, Belgium

^⋆⋆ Corresponding author; duxfrederic@gmail.com.

Received: 12 November 2024
Accepted: 15 January 2025

Abstract

We report the discovery of the first example of an Einstein zigzag lens, an extremely rare lensing configuration. In this system, J1721+8842, six images of the same background quasar are formed by two intervening galaxies, one at redshift z₁ = 0.184 and another at z₂ = 1.885. Two out of the six multiple images are deflected in opposite directions as they pass the first lens galaxy on one side and the second on the other side – the optical paths forming zigzags between the two deflectors. In this paper we demonstrate that J1721+8842, previously thought to be a lensed dual quasar, is in fact a compound lens, with the more distant lens galaxy also being distorted as an arc by the foreground galaxy. Evidence supporting this unusual lensing scenario includes: (1) identical light curves in all six lensed quasar images obtained from two years of monitoring at the Nordic Optical Telescope; (2) detection of the additional deflector at redshift z₂ = 1.885 in JWST/NIRSpec integral field unit data; and (3) a multiple-plane lens model reproducing the observed image positions. This unique configuration offers the opportunity to combine two major lensing cosmological probes, time-delay cosmography and dual source-plane lensing, since J1721+8842 features multiple lensed sources that form two distinct Einstein radii of different sizes, one of which is a variable quasar. We expect to place tight constraints on H₀ and w by combining these two probes of the same system. The z₂ = 1.885 deflector, a quiescent galaxy, is also the highest-redshift strong galaxy-scale lens with a spectroscopic redshift measurement known to date.