Thanks to the BadAstronomer I have found out about a cool new HST image from the Hubble Heritage site, what's even better is that two of the people that worked on the project to get the data work along the corridor, hence I am actually better informed than the BadAstronomer for the first time. The picture released by Hubble Heritage shows a cluster of Galaxies called Abell S0740. There are two versions of the image one without annotations and one showing zoomed regions of interest. See them both here.
This data is of particular interest because of what was found in the inner regions of the large elliptical galaxy at the centre of the cluster, when examined carefully it was discovered (by Russell Smith, here's the paper to prove it) that there were 3 gravitational arcs within the galaxy, you can see them in the inset in the top right of the image above. These arcs are the result of the light from a background galaxy being bent by the gravity of the large foreground elliptical galaxy, this has the effect of making several images of the same galaxy which are brighter and larger than they would usually be, making it possible to see galaxies further away. The gravitational arcs in this galaxy are the closest known example of strong lensing by an individual galaxy, as opposed to lensing by a cluster of galaxies.
The reason these lenses are so interesting is that when the position and brightness of the lenses are combined with the distance to the actual background galaxy (which can be worked out from its redshift) it becomes possible to measure precisely the amount of mass in the lensing galaxy within the radius of the arcs, with little or no assumptions. By looking at the way brightness of the lensing galaxy it is then possible (assuming that brightness is related to the amount of stars) to work out fraction of the mass in the lensing galaxy that is from normal matter and the fraction from dark matter. But wait that's not all.
By using the mass determined with the observed brightness of the lensing galaxy and the measured motion of stars in the galaxy (measured from spectroscopy) it becomes possible to work out exactly how the stars in the galaxy move as well, allowing a determination of parameters that can be used to constrain models of how galaxies form. Essentially these lenses could allow determinations of all the physical parameters of the lensing galaxy at accuracies previously unheard of.
Great you say, lets do it, but this is where things get difficult, it turns out that the arcs are so narrow that with current technology is actually impossible to get enough signal from them to determine a redshift, bugger. Basically they are so narrow that you add much more lensing galaxy light then you do lensed galaxy light. If STIS on the HST is ever repaired it may be possible, but if not it could be 10 years before its feasible. Note: if you happen to be someone sitting on a TAC (telescope allocation committee) and I, R. Smith or J. Lucey have a proposal to do what I've just claimed is impossible, I was wrong, we can do it, please give us the time.
Another thing I like about this work is shown in the other inset, a whole bunch of globular clusters. I study globulars, and when observing them from the ground it becomes very difficult to observe them at distances probably around 30-50Mpc away, these ones are located at 142Mpc, makes me even more annoyed that ACS has broken down, there will be no more new data like this for at least two years.