Probing Dark Matter Substructure in Lens Galaxies

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Abstract
We investigate the effects of numerous dark matter subhalos in a galaxy-sized halo on the events of strong lensing to assess their presence as expected from the cold dark matter scenario. Lens galaxies are represented by a smooth ellipsoid in an external shear field and additional cold dark matter subhalos taken from Monte Carlo realizations that accord with recent N-body results. We also consider other possible perturbers, globular clusters and luminous dwarf satellites, for comparison. We then apply the models to the particular lens systems with four images, B1422+231 and PG 1115+080, for which smooth lens models are unable to simultaneously reproduce both the positions of the images and their radio flux ratios or dust-free optical flux ratios. We show that the perturbations by both globular clusters and dwarf satellites are too small to change the flux ratios, whereas cold dark matter subhalos are the most likely perturbers to reproduce the observed flux ratios in a statistically significant manner. This result suggests to us the presence of numerous subhalos in lens galaxies, which is consistent with the results of cosmological N-body simulations.