Mapping dark matter in galaxies

Cosmological simulations can now make specific and detailed predictions for the shapes, masses, and substructure fractions in galactic dark matter halos that depend on the dark matter model assumed. Comparing these predictions to the observed mass distributions of galaxies should therefore lead to constraints on the nature of dark matter, but observable dynamical tracers can be scarce in regions where the dark matter distribution is best able to discriminate between models. One such region is the outskirts of galaxies, where the influence of baryonic matter on the dark matter halo is limited and the effect of dark substructures most prominent. New advances in instrumentation, and new surveys of Milky Way stars, are now making it possible to obtain accurate position and velocity information for the faint tidal streams, remnants of disrupted satellite galaxies, that trace out the mass distribution in the distant reaches of galaxy halos. I will demonstrate how this new information can be used to characterize the dark matter distribution in the Milky Way and in other galaxies, and thereby place constraints on the nature of dark matter.