Gemini and Hubble Space Telescope evidence for an intermediate-mass black hole in omega Centauri

The globular cluster omega 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 HSTACS 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 5 '' x 5 '' field centered on the nucleus of the cluster, as well as for a field 1400 away. We detect a clear rise in the velocity dispersion from 18.6 km s(-1) at 1400 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(-1.0)(+0.75) x 10(4) M-circle dot, 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.