Ana Bonaca1, Adrian M. Price-Whelan2, David W. Hogg3, Charlie Conroy1, Nelson Caldwell1, Phillip Cargile1, Benjamin D. Johnson1Institution(s):
1Center for Astrophysics | Harvard & Smithsonian, Cambridge, Massachusetts, United States, 2Princeton University, Princeton, New Jersey, United States, 3New York University, New York, New York, United StatesAbstract Body:
Stars escaping globular clusters form thin, long and kinematically-cold tidal streams. In pristine conditions, these streams have nearly uniform density, however, new Gaia observations of one such structure in the Milky Way halo have revealed a likely site of perturbation. The on-sky morphology suggests a recent, close encounter with a massive and dense perturber. Known baryonic objects are unlikely perturbers based on their orbital properties, but observations permit a low-mass dark-matter subhalo as a plausible candidate. This observation opens up the possibility that detailed observations of streams could measure the mass spectrum of dark-matter substructures and even identify individual substructures and their orbits in the Milky Way halo.