Probing the Dark Halo of the Milky Way
The Local Group, the regime in which detailed star-by-star studies can be done, is becoming a major testbed for the cold dark matter-based model of the Universe. Critical to these tests is a better estimate of the total Milky Way halo mass. The most reliable means by which to constrain the properties of the Milky Way dark halo is through assessing the 6-D phase space distributions of tracers of its gravitational potential. This requires accurate proper motions (tangential velocities) in addition to generally known radial velocities for field stars, streams, and satellite galaxies widely distributed throughout the halo. In turn, these measurements allow us to investigate the past histories of these tracers, and thus the accretion history of the Milky Way. I will discuss some novel approaches we have been developing to obtain proper motions for a variety of tracers in the Milky Way halo, including streams, globular clusters, and satellite galaxies, to definitively constrain the Milky Way's dark halo mass, shape and distribution. These techniques involve space-based imagers (HST), wide-field ground-based imagers, as well as imagers equipped with adaptive optics systems. I will also discuss the future prospects for this work, and the trade-off between Gaia, space missions (JWST, WFIRST, EUCLID), and ELTs.