Abstract

Stars orbiting in the halo of our galaxy, the Milky Way, are a window into the distribution of dark matter. Tidally disrupting star clusters are especially valuable tracers, because in pristine conditions they produce thin stellar streams of nearly uniform density. I will present maps of stellar streams based on the latest photometric and astrometric data that reveal variations in the width and density of streams—typical signatures of dynamical perturbation. Dynamical modeling of a perturbed stream GD-1 suggests it recently had a close encounter with a massive and dense perturber, while precise radial velocities constrain the perturber’s orbit and present-day location. Known baryonic objects are unlikely perturbers based on their orbital properties, but observations permit a low-mass dark-matter subhalo as a plausible candidate. Data being delivered by big ground-based surveys of this decade (e.g., DESI , LSST) will enable such studies in hundreds of stellar streams. I will discuss how the upcoming observations can be used to measure the mass spectrum of dark-matter substructures and even identify individual substructures and their orbits in the Milky Way halo.