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 radio source
imaged into a ring and two compact, flat-spectrum components separated by 2.1
arcsec. 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(lens) = 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 Angstroms, which
we associate with CIII] 1909 from a type-2 quasar at z(source) = 1.849. Support
for this redshift identification comes from weaker emission associated with CIV
1549 and HeII 1640, 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 ~1e-13 erg/cm2/s and a
best-fit column density of ~3e22 /cm2. 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.
