On the central structure of M15
  We present a detailed comparison between the latest observational data on the kinematical structure of the core of M15, obtained with the Hubble STIS and WFPC2 instruments, and the results of dynamical simulations carried out using the special-purpose GRAPE-6 computer. The observations imply the presence of a significant amount of dark matter in the cluster core. In our dynamical simulations, neutron stars and/or massive white dwarfs concentrate to the center through mass segregation, resulting in a sharp increase in $M/L$ toward the center. While consistent with the presence of a central black hole, the Hubble data can also be explained by this central concentration of stellar-mass compact objects. The latter interpretation is more conservative, since such remnants result naturally from stellar evolution, although runaway merging leading to the formation of a black hole may also occur for some range of initial conditions. We conclude that no central massive object is required to explain the observational data, although we cannot conclusively exclude such an object at the level of $\sim500-1000$ solar masses. Our findings are unchanged when we reduce the assumed neutron-star retention fraction in our simulations from 100% to 0%. 
H. Baumgardt, P. Hut, J. Makino, S. McMillan, S. Portegies Zwart