The Cosmic Horseshoe: Discovery of an Einstein Ring around a Giant Luminous Red Galaxy

We report the discovery of an almost complete (~300°) 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.