Water Masers in the Andromeda Galaxy: The First Step Toward Proper
  Motion
  We have detected and confirmed five water maser complexes in the Andromeda
Galaxy (M31) using the Green Bank Telescope. These masers will provide the high
brightness temperature point sources needed for proper motion studies of M31,
enabling measurement of its full three-dimensional velocity vector and its
geometric distance via proper rotation. The motion of M31 is the keystone of
Local Group dynamics and a gateway to the dark matter profiles of galaxies in
general. Our survey for water masers selected 206 luminous compact 24
micron-emitting regions in M31 and was sensitive enough to detect any maser
useful for ~10 microarcsecond per year astrometry. The newly discovered masers
span the isotropic luminosity range 0.3-1.9 x 10^-3 L(Sun) in single spectral
components and are analogous to luminous Galactic masers. The masers are
distributed around the molecular ring, including locations close to the major
and minor axes, which is nearly ideal for proper motion studies. We find no
correlation between 24 micron luminosity and water maser luminosity, suggesting
that while water masers arise in star-forming regions, the nonlinear
amplification pathways and beamed nature of the water masers means that they
are not predictable based on IR luminosity alone. This suggests that there are
additional bright masers to be found in M31. We predict that the geometric
distance and systemic proper motion of M31 can be measured in 2-3 years with
current facilities. A "moving cluster" observation of diverging masers as M31
approaches the Galaxy may be possible in the long term.
