The cosmic horseshoe: Discovery of an Einstein ring around a giant luminous red galaxy

We report the discovery of an almost complete (similar to 300 degrees) Einstein ring of diameter 10 '' in Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5). Spectroscopic data from the 6 m telescope of the Special Astrophysical Observatory reveal that the deflecting galaxy has a line-of-sight velocity dispersion in excess of 400 km s(-1) and a redshift of 0.444, while the source is a star- forming galaxy with a redshift of 2.379. From its color, luminosity, and velocity dispersion, we argue that this is the most massive galaxy lens hitherto discovered.