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A. J. Romanowsky (Kapteyn Institute)
Progress on the understanding of the halo dynamics in elliptical galaxies has lagged behind that in spiral galaxies owing to the general lack of simple, easily-observable dynamical tracers in the former. But this situation is now being remedied with the routine observation of high-resolution stellar line-of-sight velocity distributions (LOSVDs) and of discrete object velocities at large radii (globular clusters and planetary nebulae), in combination with improved dynamical modeling techniques.
We investigate the dynamics of the giant elliptical galaxy M87. The data include higher-order moments of the stellar LOSVD, and the discrete velocities of over 200 globular clusters. These provide dynamical constraints out to 5 effective radii (40 kpc). Our spherical models allow for full freedom of variations in the orbital anisotropy; we introduce an extension to orbit-based modeling techniques that fits the discrete velocity data using maximum likelihood.
For this galaxy, we are able to strongly rule out a constant mass-to-light ratio. We find that the total mass falls off more slowly than r-2 in the outer parts, suggesting that the core of the dark halo of the Virgo Cluster itself is being detected.
The author(s) of this abstract have provided an email address for comments about the abstract: romanow@astro.rug.nl