| Despite decades of detailed observations and analysis, the basic structural and kinematic properties of the Milky Way and Andromeda (M31) galaxies remain uncertain. For example, published values for the masses of the two galaxies vary by factors of a few and there is disagreement over which of the two galaxies is the more massive. We present new mass estimates of the Andromeda galaxy which indicate it is likely that M31 and the Milky Way have approximately the same mass. We use the kinematics of the dwarf satellite galaxies and self-consistent disk-bulge-halo models of M31 to constrain the extent and mass of the galaxy. The data set includes the line of sight distances and velocities of 15 satellite galaxies. We find a most likely mass of 16.7 x 1011 M⊙ , and a range in total mass of (5.8-20.0) x 1011 M⊙ . |
