Fredagskollokvium: Re-runs and pearl strings: Some highlights from HST- and ground-based follow-up of the Sloan Giant Arcs Survey.

During the course of the Sloan Giant Arcs Survey (SGAS) we have identified 270 new gravitational lens systems 
where clusters of galaxies act as strong lenses. The typical lensed sources we find are star-forming galaxies 
in the redshift range 1<z<3. Because of the lensing magnification, some of these sources are among the 
brightest galaxies ever seen at these redshifts. The colloquium will describe follow-up studies with the Hubble Space Telescope 
of some of the most interesting lens systems discovered so far, and what we have learnt and can learn from them: 
In one case, a distant lensed quasar is split into six distinct images by an intervening cluster lens.     
An ongoing monitoring campaign with the Nordic Optical Telescope has so far allowed the determination of 
two time delays in this system and demonstrated how this system will be a very rich source of information 
about both the lensed quasar and the cluster lens itself. In another lens system, the lens itself shows a highly unusual    
mode of “pearls-on-a-string” star formation caused by an ongoing collision of two giant elliptical galaxies. In a third system, 
we can resolve star forming regions in a source galaxy, looking back to an epoch when the universe was 20% of its present age.