Fredagskollokvium: Håkon Dahle: SDSS J2222+2745 - A lens that should not exist ? Discovery of an exceptionally rare gravitational lens.

I report on the recent discovery of an exceptionally rare gravitational lens system, producing five spectroscopically confirmed images of a quasar lensed by a foreground galaxy cluster. There is also strong photometric and spectroscopic evidence for a sixth lensed image of the same quasar. Both the large image separations and the high image multiplicity of the lensed quasar are in themselves very rare, and observing the combination of these two factors is an exceptionally unlikely occurrence in present data sets. The lens system was discovered in the course of the Sloan Giant Arcs Survey, in which candidate gravitational lenses are identified in the Sloan Digital Sky Survey and targeted for follow-up and verification with the Nordic Optical Telescope.