Globular clusters

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 van der Marel & Anderson 2009). True evidence of an IMBH will require advancement in both data as well as modeling, like directly fitting discrete data instead of first spatially binning (Watkins, van de Ven, et al. 2013). Also, whereas M15 as proto-typical core-collapsed GC was thought to be a good candidate for an IMBH, it is now clear that the opposite is actually true: the presence of an IMBH would have prevented the high central concentration of dark remnants that naturally explains the dynamically inferred high mass-to-light ratio (den Brok, van de Ven, et al. 2013).


On the other hand, the high central concentration in M15 due to strong mass segregation provides an environment for close encounters between stars and their (compact) remnants, which in turn might be related with phenomena such as short gamma-ray bursts (Lee, Ramirez-Ruiz & van de Ven 2010).