An improved mass determination for M31 from its satellite galaxies

The High-Resolution Echelle Spectrometer on the Keck I telescope has been used to measure the first radial velocities for stars belonging to the Andromeda I and III dwarf spheroidal galaxies. Our radial velocity for And III matches that reported by Blitz & Robishaw for an adjacent H I gas complex, supporting the association of this galaxy with a high-velocity cloud. New and previously published radial velocities for a sample of confirmed or suspected M31 satellites are combined with a homogeneous set of distance estimates to calculate the total mass of M31. Assuming the satellite orbits are isotropic, we find a median mass of M_A ≃ (7.9 ± 0.5) × 10¹¹ M_⊙ from eight candidate satellites having deprojected distances from M31 in excess of R_A ≃ 100 kpc. If the orbits are radial, the inferred mass increases to M_A ≃ (21.5 ± 3.8) × 10¹¹ M_⊙ for circular orbits, the enclosed mass is M_A ≃ (3.7 ± 0.4) × 10¹¹ M_⊙. These masses are somewhat lower than those found for luminous spiral galaxies based on the ensemble dynamics of the their satellites, although the estimates are nevertheless consistent given the large uncertainties.