The globular cluster ω Centauri is one of the largest and most massive members of the galactic system. However, its classification as a globular cluster has been challenged making it a candidate for being the stripped core of an accreted dwarf galaxy; this together with the fact that it has one of the largest velocity dispersions for star clusters in our galaxy makes it an interesting candidate for harboring an intermediate mass black hole. We measure the surface brightness profile from integrated light on an HST /ACS image of the center, and find a central power-law cusp of logarithmic slope -0.08. We also analyze Gemini GMOS-IFU kinematic data for a 5x5 ′′ field centered on the nucleus of the cluster, as well as for a field 14 ′′ away. We detect a clear rise in the velocity dispersion from 18.6 km s −1 at 14 ′′ to 23 km s −1 in the center. A rise in the velocity dispersion could be due to a central black hole, a central concentration of stellar remnants, or a central orbital structure that is radially biased. We discuss each of these possibilities. An isotropic, spherical dynamical model implies a black hole mass of 4.0 +0.75 −1.0 × 10 4 M ⊙ , and excludes the no black hole case at greater than 99% significance. We have also run flattened, orbit-based models and find similar results. While our preferred model is the existence of a central black hole, detailed numerical simulations are required to confidently rule out the other possibilities.

The Hubble Space Telescope (HST ) allows us to study the central surface brightness profiles of globular clusters at unprecedented detail. We have mined the HST archives to obtain 38 WFPC2 images of Galactic globular clusters with adequate exposure times and filters, which we use to measure their central structure. We outline a reliable method to obtain surface brightness profiles from integrated light that we test on an extensive set of simulated images. Most clusters have central surface brightness about 0.5 mag brighter than previous measurements made from ground-based data, with the largest differences around 2 mag. Including the uncertainties in the slope estimates, the surface brightness slope distribution is consistent with half of the sample having flat cores and the remaining half showing a gradual decline from 0 to À0.8 [d log AE/d log r)]. We deproject the surface brightness profiles in a nonparametric way to obtain luminosity density profiles. The distribution of luminosity density logarithmic slopes shows similar features, with half of the sample between À0.4 and À1.8. These results are in contrast to our theoretical bias that the central regions of globular clusters are either isothermal (i.e., flat central profiles) or very steep (i.e., luminosity density slope approximately À1.6) for core-collapse clusters. With only 50% of our sample having central profiles consistent with isothermal cores, King models appear to represent most globular clusters in their cores poorly.

The Galactic globular cluster ω Centauri is a prime candidate for hosting an intermediate mass black hole. Recent measurements lead to contradictory conclusions on this issue. We use VLT-FLAMES to obtain new integrated spectra for the central region of ω Centauri. We combine these data with existing measurements of the radial velocity dispersion profile taking into account a new derived center from kinematics and two different centers from the literature. The data support previous measurements performed for a smaller field of view and show a discrepancy with the results from a large proper motion data set. We see a rise in the radial velocity dispersion in the central region to 22.8±1.2 km s −1 , which provides a strong sign for a central black hole. Isotropic dynamical models for ω Centauri imply black hole masses ranging from 3.0 to 5.2 × 10 4 M ⊙ depending on the center. The best-fitted mass is (4.7 ± 1.0) × 10 4 M ⊙ .