A candidate optical Einstein ring

We present spectroscopic and imaging observations of a candidate optical Einstein ring. The deflector is a luminous early-type galaxy, M(BT) = -20.9 + 5logh (q(0) = 0.1), at redshift z = 0.485. Superposed on the otherwise normal optical spectrum of the galaxy is a strong emission feature at 5588 Angstrom. No other emission lines are evident over the wavelength range lambda lambda 4456-7912 Angstrom. Narrow-band imaging of the system at the emission-line wavelength reveals a semicircular are, radius r = 1.35 arcsec, centred on the peak of the galaxy surface brightness distribution. The properties of the system can be reproduced by a gravitational lens model in which the deflector has a constant mass-to-light ratio (corrected for evolution) of M/L(B(0)) similar to 20h and the source is an intrinsically small, but resolved, object at high redshift, either at z = 3.597 (if the line is Lya lambda 1216), or at z = 2.607 (if the line is C IV lambda 1549). Alternatives to the lensing explanation, in which the emission feature is associated with [O II] lambda 3727, with a velocity difference of 2900 km s(-1) relative to the absorption-line redshift of the galaxy, appear to be extremely improbable. Unambiguous identification of the system as a gravitational lens requires the detection of a second emission line, and would confirm the system as the first Einstein ring to be discovered in the optical.