The Inner Velocity Dispersion Profile of M15

Previous abstract Next abstract

Session 58 -- The Galaxy
Display presentation, Thursday, 2, 1994, 9:20-6:30

[58.14] The Inner Velocity Dispersion Profile of M15

J.D. Dull, H.N. Cohn, P.M. Lugger (Indiana U.), B.W. Murphy (Butler U.), P.O. Seitzer (U. of Michigan), P.J. Callanan (CfA), R.G.M. Rutten (La Palma Obs.), P.A. Charles (Oxford U.)

We report a new measurement of the velocity dispersion profile within $1'$ (3 pc) of the center of the globular cluster M15 (NGC 7078), using long-slit spectra from the 4.2-m William Herschel Telescope at La Palma Observatory. Spectra for a total of 23 slit positions were obtained during two observing runs. Each run used a set of parallel positions to map out the central region of the cluster; the position angle used during the second run was orthogonal to that used for the first. The spectra are centered near the Ca~II infrared triplet at 8650~\AA, with a spectral range of 400~\AA.

We determined radial velocities by cross-correlation techniques for 130 cluster members. A total of 32 stars were observed more than once. Internal and external comparisons indicate a velocity accuracy of about 4~\mbox{km~s$^{-1}$}. The velocity dispersion profile rises from about $\sigma=8~\mbox{km~s$^{-1}$}$ near $1'$ from the center of the cluster to about $\sigma=12~\mbox{km~s$^{-1}$}$ at $20''$. Inside of $20''$ the dispersion remains approximately constant with no evidence for a sharp rise near the center. This last result stands in contrast with that of Peterson et al. \ (1989, ApJ, 347, 251) who found a central velocity dispersion of $25\pm7~\mbox{km~s$^{-1}$}$, based on a line broadening measurement. Our velocity dispersion profile is in good agreement with those determined in the recent studies of Gebhardt et al. \ 1994 (ApJ, in press) and Dubath~\& Meylan (1994, A\&A, in press).

The behavior of the central velocity dispersion profile of M15 is consistent with the predictions of Fokker-Planck models developed by Grabhorn et al. \ (1992, ApJ, 392, 86) and Phinney (1993, ASP Conf.\ Ser., 50, 141) for globular clusters undergoing core collapse. These models predict the presence of central populations of a few $\times 10^4$ degenerate remnants with masses exceeding about $1~M_\odot$, of which a few $\times 10^3$ are $1.4~M_\odot$ neutron stars. There appears to be no need to invoke the presence of a massive central black hole in M15.

Thursday program listing