There have been many optical giant arcs discovered, caused by the lensing effects of massive galaxy clusters and their central galaxies. But, few optical rings have ever been found, despite theoretical predictions that they should be abundant. Using data from the
Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5), astronomers found the largest optical Einstein ring known, an almost complete (~300°) Einstein ring of diameter 10.2", more than 5 times the size of previously-known optical rings. Further imaging follow-up with the Wide Field Camera on the INT revealed a "horseshoe" shape with three brightness peaks, and thus the name "Cosmic Horseshoe".
The lensing galaxy itself is an interesting object: It's a member of a rare population of Luminous Red Galaxies (LRGs). These are the largest and most massive galaxies in the universe, and they are also believed to host massive black holes. This 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, they argue that this is the most massive galaxy lens hitherto discovered.
INT WFC u, g, i composite of the Cosmic Horseshoe from follow-up data. (Extracted from Belokurov et al., 2007, ApJ, 671, L9) [ JPEG | TIFF ].