J1721+8842: a gravitationally lensed binary quasar with a proximate damped Lyman-α absorber^⋆

¹ Institute of Physics, Laboratory of Astrophysics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
e-mail: cameron.lemon@epfl.ch
² STAR Institute, Quartier Agora, Allée du six Août, 19c, 4000 Liège, Belgium
³ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁴ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁵ DARK, Niels Bohr Institute, Jagtvej 128, 2200 Copenhagen, Denmark

Received: 2 September 2021
Accepted: 29 October 2021

Abstract

High-redshift binary quasars provide key insights into mergers and quasar activity, and are useful tools for probing the spatial kinematics and chemistry of galaxies along the line of sight. However, only three sub-10-kpc binaries have been confirmed above z = 1. Gravitational lensing would provide a way to easily resolve such binaries, study them in higher resolution, and provide more sightlines, though the required alignment with a massive foreground galaxy is rare. Through image deconvolution of StanCam Nordic Optical Telescope (NOT) monitoring data, we reveal two further point sources in the known, z ≈ 2.38, quadruply lensed quasar (quad) J1721+8842. An ALFOSC/NOT long-slit spectrum shows that the brighter of these two sources is a quasar with z = 2.369 ± 0.007 based on the C III] line, while the C III] redshift of the quad is z = 2.364 ± 0.003. Lens modelling using point-source positions rules out a single source model, favouring an isothermal lens mass profile with two quasar sources separated by ∼6.0 kpc (0.73″) in projection. Given the resolving ability from lensing and current lensed quasar statistics, this discovery suggests a large population of undiscovered, unlensed sub-10-kpc binaries. We also analyse spectra of two images of the quad, showing narrow Lyα emission within the trough of a proximate damped Lyman-α absorber (PDLA). An apparent mismatch between the continuum and narrow line flux ratios provides a new potential tool for simultaneously studying microlensing and the quasar host galaxy. Signs of the PDLA are also seen in the second source, but a deeper spectrum is still required to confirm this. Thanks to the multiple lines of sight from lensing and two quasar sources, this system offers simultaneous subparsec- and kiloparsec-scale probes of a PDLA.