Institute for Advanced Study Informal Astrophysics Seminar

Galaxy clusters as astronomical tools generally offer two advantages: to boost the brightnesses of objects in the background, and to study the dark matter in the lens. We can now introduce a third use: to unveil properties of the underlying dark matter by the detection of caustic crossing events which can yield magnification factors of 10,000 or more. We review the approach to detect a new set of massive objects such as galaxy clusters by their rest-frame far-infrared colors (and not by the Sunyaev-Z'eldovich effect). We move on to present Hubble Space Telescope WFC3-IR imaging in the fields of six classical giant arcs discovered using a combination of Planck and Herschel space facilities. We construct lensing models from the many examples of image multiplicities, and compare our results with galaxy over-densities identified by more conventional methods. We conclude with a discussion on the emerging field of caustic crossings, for which we address the question of which are the very best giant arcs for the detection of microlensing events in high magnification regions.