A Dynamical N-body model for the central region of ω Centauri

¹ European Southern Observatory (ESO), Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
e-mail: bjalali@ph1.uni-koeln.de
² Argelander Institute für Astronomie (AIfA), Auf dem Hügel 71, 53121 Bonn, Germany
³ University of Queensland, School of Mathematics and Physics, 4072 Brisbane, Australia
⁴ Astronomy Department, University of Texas at Austin, Austin TX 78712, USA
⁵ University Observatory, Ludwig Maximilians University, 81679 Munich, Germany
⁶ Instituto de Astronomia, Universidad Nacional Autonoma de Mexico (UNAM), A.P. 70-264, 04510 Mexico, Mexico
⁷ Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands

Received: 18 March 2011
Accepted: 7 November 2011

Abstract

Context. Supermassive black holes (SMBHs) are fundamental keys to understand the formation and evolution of their host galaxies. However, the formation and growth of SMBHs are not yet well understood. One of the proposed formation scenarios is the growth of SMBHs from seed intermediate-mass black holes (IMBHs, 10² to 10⁵ M_⊙) formed in star clusters. In this context, and also with respect to the low mass end of the M_• − σ relation for galaxies, globular clusters are in a mass range that make them ideal systems to look for IMBHs. Among Galactic star clusters, the massive cluster ω Centauri is a special target due to its central high velocity dispersion and also its multiple stellar populations.

Aims. We study the central structure and dynamics of the star cluster ω Centauri to examine whether an IMBH is necessary to explain the observed velocity dispersion and surface brightness profiles.

Methods. We perform direct N-body simulations on GPU and GRAPE special purpose computers to follow the dynamical evolution of ω Centauri. The simulations are compared to the most recent data-sets in order to explain the present-day conditions of the cluster and to constrain the initial conditions leading to the observed profiles.

Results. We find that starting from isotropic spherical multi-mass King models and within our canonical assumptions, a model with a central IMBH mass of 2% of the cluster stellar mass, i.e. a 5. × 10⁴ M_⊙ IMBH, provides a satisfactory fit to both the observed shallow cusp in surface brightness and the continuous rise towards the center of the radial velocity dispersion profile. In our isotropic spherical models, the predicted proper motion dispersion for the best-fit model is the same as the radial velocity dispersion one.

Conclusions. We conclude that with the presence of a central IMBH in our models, we reproduce consistently the rise in the radial velocity dispersion. Furthermore, we always end up with a shallow cusp in the projected surface brightness of our model clusters containing an IMBH. In addition, we find that the M/L ratio seems to be constant in the central region, and starts to rise slightly from the core radius outwards for all models independent of the presence of a black hole. Considering our initial parameter space, it is not possible to explain the observations without a central IMBH for ω Centauri. To further strengthen the presence of an IMBH as a unique explanation of the observed light and kinematics more detailed analysis such as investigating the contribution of primordial binaries and different anisotropy profiles should be studied.