0 Assigned ID: 1226 ----- Attempting Submission 1 (Thu Feb 28 16:08:10 GMT 2013) ----- The Milky Way (MW) is shrouded in a faint metal-poor stellar halo. Its structure and kinematics provide a unique archaeological record of the MW's formation, past evolution, and accretion history, and they also constrain the total dark matter mass out to large radii. The stellar density profile and radial velocity dispersion profile of the halo are known, but our understanding of the halo is limited by a striking lack of knowledge about the transverse motions of its stars. It is difficult from the ground to determine proper motions (PMs) far outside of the solar neighborhood. We have recently developed techniques for PM measurements from multi-epoch HST data using distant background galaxies as absolute reference frame. These techniques can be used to identify and measure PMs of distant halo stars in random fields: from the 3 fields observed in our widely-publicized Andromeda galaxy PM program, we managed to isolate 13 halo stars located at about 25 kpc. Their PMs imply a radial gradient in the halo velocity ellipsoid that suggests we are looking through a shell created by a past accretion event (NASA/STScI press release 2013-07). We propose to analyze another 142 fields that have deep multi-epoch data in the HST Archive for which we can perform similar PM measurements. We will create Legacy catalogs, and will extract PMs for ~710 distant MW halo stars (incl. 100 beyond 50 kpc). This will yield a spectacular improvement in our understanding of the halo, and of the structure, past evolution and mass of the MW. The proposed program is a prime example of how the HST Archive can be used after 23 years on orbit to tackle totally new forefront science. RESOLVED STELLAR POPULATIONS Default Legacy false false false false false SUPPORTED