Dynamical model of the globular cluster omega Cen. Left: The stars in omega Cen with proper motion measurements (top) and line-of-sight velocity measurements (bottom), that are used to construct the dynamical model with Schwarzschild's orbit-superposition method. Right: Mean velocity and velocity dispersion calculated from these discrete kinematic measurements (first and third column), and from the best-fit dynamical model (second and fourth column). The comparison between data and models provides accurate determinations of the distance, inclination and mass-to-light ratio of omega Cen. In addition, Schwarzschild's method also yields the orbital weight distribution, which reveals signatures of tidal interaction and also a central stellar disk. While this phase-space structure together with the multiple stellar populations in omega Cen is unexpected for a globular cluster, it does support its proposed origin as the nucleus of a stripped dwarf galaxy. All this supports its proposed origin as the nucleus of a stripped dwarf galaxy. (See van de Ven et al. 2006 for the orbital weight distribution and further details.)

Globular clusters (GCs) are often considered to be simple, spherical systems with an old single stellar population, but improving observations and modeling reveal a more complex formation history. A striking example is omega Cen, with multiple stellar populations that also show up as a puzzling double main sequence in Hubble Space Telescope observations (Bedin et al. 2004). In van de Ven et al. (2006; see also figure above), we present a detailed dynamical model of this GC, using an extension of Schwarzschild's orbit superposition method applied to line-of-sight velocities and proper motion measurements of thousands of stars in omega Cen. The intrinsic orbital structure not only shows a clear signature of tidal interaction, but also a central stellar disk. All this supports its proposed origin as the nucleus of a stripped dwarf galaxy.

It is then perhaps also less surprising that there are claims of an intermediate-massive black hole (IMBH) in omega Cen (Noyola, Gebhardt & Bergmann 2008; but see Anderson \& van der Marel 2009, and van der Marel \& Anderson 2009) as well as its analogue G1 in Andromeda (Gehbardt, Rich & Ho 2005, Ulvestad, Greene & Ho 2007). However, true evidence for an IMBH requires more and better (kinematic) data such as Hubble Space Telescope proper motions, as well as more general and realistic (dynamical) modeling such like with the above Schwarzschild's method. In van den Bosch et al. (2006), we apply the latter method to line-of-sight velocities and proper motion data of the (genuine) GC M15. Although the formally best-fit model includes an IMBH, we cannot exclude a model in which dark remnants in the collapsed core explain the central peak in the observed light and velocity dispersion profile.

Apart from the nucleus of some galaxies, perhaps only in the collapsed cores of GCs such as M15 the densities can become high enough for close encounters between stars and their (compact) remnants. Combining the encounter rate with the distribution of GCs in galaxies, we are investigating the frequency and evolution of these close encounters and their possibly relation with phenomena such as short gamma-ray bursts (Lee, Ramirez-Ruiz & van de Ven 2010).