A Redshift for the First Einstein Ring, MG 1131+0456

MG 1131+0456 is a radio-selected gravitational lens, and is the first known Einstein ring. Discovered in 1988, the system consists of a bright (S_{74 MHz} = 3.7 Jy) radio source imaged into a ring and two compact, flat-spectrum components separated by 2"1. The ring is optically faint (R = 23.3), rising steeply into the near- and mid-infrared (K = 17.8; W2 = 13.4). The system has been intensively studied in the intervening years, including high-resolution radio imaging, radio monitoring, and near-infrared imaging with Hubble and Keck. The lensing galaxy is at z_l = 0.844. However, to date, no spectroscopic redshift had been reported for the lensed source. Using archival Keck data from 1997, we report the robust detection of a single narrow emission line at 5438 Å, which we associate with C III] λ1909 Å from a type-2 quasar at z_s = 1.849. Support for this redshift identification comes from weaker emission associated with C IV λ1549 Å and He II λ1640 Å, typical of type-2 quasars, as well as the lack of emission lines in archival near-infrared Keck spectroscopy. We also present, for the first time, Cycle 1 Chandra observations of MG 1131+0456, which clearly resolves into two point sources with a combined flux of ∼10^-13 erg cm^-2 s^-1 and a best-fit column density of ∼3 × 10²² cm^-2. We suggest a new method to identify candidate lensed active galactic nuclei from low-resolution X-ray surveys such as eROSITA by targeting sources that have anomalously high X-ray luminosity given their mid-infrared luminosity.