The Transverse velocity of the Andromeda system, derived from the M31
  satellite population
  We present a dynamical measurement of the tangential motion of the Andromeda
system, the ensemble consisting of the Andromeda Galaxy (M31) and its
satellites. The system is modelled as a structure with cosmologically-motivated
velocity dispersion and density profiles, and we show that our method works
well when tested using the most massive substructures in high-resolution Lambda
Cold Dark Matter simulations. Applied to the sample of 40 currently-known
galaxies of this system, we find a value for the transverse velocity of 164.4
+/- 61.8 km/s (v_East = -111.5 +/- 70.2 km/s and v_North = 99.4 +/- 60.0 km/s),
significantly higher than previous estimates of the proper motion of M31
itself. This result has significant implications on estimates of the mass of
the Local Group, as well as on its past and future history.
