Motions of Galaxies in the Local Group and Beyond: an Astro2010 Science
  White Paper
  Recent advances with the VLBA have resulted in ~10 micro-arcsec astrometry
for compact sources in external galaxies, and measurement of the proper motion
of Local Group galaxies has been demonstrated. With improved telescopes and
equipment, we could greatly improve upon and expand these measurements,
including a measurement of the proper motion of the Andromeda galaxy, which is
key to understanding the history and fate of the Local Group. The combination
of optical velocities and radio astrometric data would allow detailed modeling
of the mass distributions of the disks, bulges, and dark matter halos of
galaxies in clusters.
