Probing dark matter substructure with lensed quasar narrow-line emission
Strong gravitational lensing provides a powerful test of Cold Dark Matter (CDM) as it enables the detection and mass measurement of low mass haloes even if they do not contain baryons. Gravitational lensing tests of CDM have been limited by the small number of systems which have significant radio emission, which is extended enough avoid significant lensing by stars in the plane of the lens galaxy, and red enough to be minimally affected by differential dust extinction. Narrow-line emission is a promising alternative as it is also extended and, unlike radio, detectable in virtually all optically selected quasar lenses, and thus naturally applicable to the tens of systems expected to be found in DES and the ~hundreds in LSST. I will present results from two observing programs to use narrow-line lensing to detect dark matter substructure. I will discuss constraints from the currently known sample of quasar lenses as well as from the future lenses to be discovered in DES and LSST.