Dynamical mass estimates for the halo of M31 from Keck spectroscopy

The last few months have seen the measurements of the radial velocities of all of the dwarf spheroidal companions to the Andromeda galaxy (M31) using the spectrographs (HIRES and LRIS) on the Keck Telescope. This Letter summarizes the data on the radial velocities and distances for all the companion galaxies and presents new dynamical modeling to estimate the mass of the extended halo of M31. The best-fit values for the total mass of M31 are similar to(7-10) x 10(11) M., depending on the details of the modeling. The mass estimate is accompanied by considerable uncertainty caused by the small size of the data set; for example, the upper bound on the total mass is similar to 24 x 10(11) M., while the lower bound is similar to 3 x 10(11) M-.. These values are less than the most recent estimates of the most likely mass of the Milky Way halo. Bearing in mind all the uncertainties, a fair conclusion is that the M31 halo is roughly as massive as that of the Milky Way halo. There is no dynamical evidence for the widely held belief that M31 is more massive-it may even be less massive.