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 paper summarises the data on the radial velocities and distances for all the companion galaxies and presents new dynamical modelling to estimate the mass of extended halo of M31. The best fit values for the total mass of M31 are ∼ 7− 10× 10 M⊙, depending on the details of the modelling. The mass estimate is accompanied by considerable uncertainty caused by the smallness of the dataset; for example, the upper bound on the total mass is ∼ 24 × 10 M⊙, while the lower bound is ∼ 3 × 10 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. Subject headings: galaxies: individual M31 – galaxies: kinematics and dynamics – Local Group – galaxies: structure – galaxies: halos