The shape of the mass distribution in M31 from its globular cluster system

The velocity dispersion and rotation velocity of the M31 globular cluster system depend on the relative division of mass between the flat disk and a spherically symmetric halo. Using the tensor virial theorem, it is shown in detail how the mass ratio can be constrained. Radial velocities have been collected for 149 globular clusters in M31. With no assumptions about the isotropy of the velocity distribution, the globular cluster kinematics are consistent with the mass distribution inferred by the rotation curve but otherwise place no strong constraints on the relative division of the mass. If the velocity distribution is isotropic, models with the disk mass ranging between 1/2 and 1 times its maximum possible value are marginally favored.