The Dynamical Mass-to-Light Ratio Profile and Distance of the Globular Cluster M15

We construct orbit-based axisymmetric dynamical models for the globular cluster M15 that fit ground-based line-of-sight velocities and Hubble Space Telescope line-of-sight velocities and proper motions. This allows us to constrain the variation of the mass-to-light ratio M/L as a function of radius in the cluster and to measure the distance and inclination of the cluster. We obtain a best-fitting inclination of 60° ± 15°, a dynamical distance of 10.3 ± 0.4 kpc, and an M/L profile with a central peak. The inferred mass in the central 0.05 pc is 3400 M☉, implying a central density of at least 7.4 × 106 M☉ pc-3. We cannot distinguish the nature of the central mass concentration. It could be an intermediate mass black hole, or it could be a large number of compact objects, or it could be a combination. The central 4'' of M15 appears to contain a rapidly spinning core, and we speculate on its origin.

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