Gravitational imaging of cold dark matter substructures

We propose the novel method of `gravitational imaging' to detect and quantify luminous and dark matter substructures in gravitational lens galaxies. The method utilizes highly magnified Einstein rings and arcs as sensitive probes of small perturbations in the lens potential (due to the presence of the mass substructure), reconstructing the gravitational lens potential non-parametrically. Numerical simulations show that the implemented algorithm can reconstruct the smooth mass distribution of a typical lens galaxy - exhibiting reasonable signal-to-noise Einstein rings - as well as compact substructure with masses as low as M<sub>sub</sub>~ 10<sup>-3</sup>M<sub>lens</sub>, if present. `Gravitational imaging' of pure dark matter substructures around massive galaxies can provide new insight into the standard cold dark matter paradigm, using very different physics than ground-based direct-detection experiments, and probe the hierarchical structure-formation model which predicts that this substructure exists in great abundance.