Campus, Varian 355
Gilman: Strong gravitational lensing by galaxies provides a unique and powerful means of probing dark-matter structure on sub-galactic scales. Below 10^9 solar masses, various models based on dark-matter theory make unique predictions for the abundance and density profiles of both subhalos associated with the main deflector and field halos along the line of sight. These populations of low-mass structures alter the magnification ratios between the four images of a background quasar produced in strong gravitational lenses, providing a direct avenue to probe structure on scales where halos are mostly, if not completely, dark. I will describe recent work that uses a novel forward modeling method to extract information from strong lens systems, placing stringent constraints on the free-streaming length of dark matter, and the first constraint on the mass-concentration relation of cold dark matter halos on sub-galactic scales across cosmological distance.