Central kinematics of the globular cluster NGC 2808: upper limit on the mass of an intermediate-mass black hole^⋆,⋆⋆

¹ European Southern Observatory (ESO), Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
e-mail: nluetzge@eso.org
² Astronomy Department, University of Texas at Austin, Austin, TX 78712, USA
³ School of Mathematics and Physics, University of Queensland, Brisbane, QLD 4072, Australia
⁴ Instituto de Astronomia, Universidad Nacional Autonoma de Mexico (UNAM), AP 70-264, 04510 Mexico
⁵ University Observatory, Ludwig Maximilians University, 81679 Munich, Germany
⁶ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁷ Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands

Received: 10 April 2012
Accepted: 18 April 2012

Abstract

Context. Globular clusters are an excellent laboratory for stellar population and dynamical research. Recent studies have shown that these stellar systems are not as simple as previously assumed. With multiple stellar populations as well as outer rotation and mass segregation they turn out to exhibit high complexity. This includes intermediate-mass black holes (IMBHs) which are proposed to sit at the centers of some massive globular clusters. Today’s high angular resolution ground based spectrographs allow velocity-dispersion measurements at a spatial resolution comparable to the radius of influence for plausible IMBH masses, and to detect changes in the inner velocity-dispersion profile. Together with high quality photometric data from HST, it is possible to constrain black-hole masses by their kinematic signatures.

Aims. We determine the central velocity-dispersion profile of the globular cluster NGC 2808 using VLT/FLAMES spectroscopy. In combination with HST/ACS data our goal is to probe whether this massive cluster hosts an IMBH at its center and constrain the cluster mass to light ratio as well as its total mass.

Methods. We derive a velocity-dispersion profile from integral field spectroscopy in the center and Fabry Perot data for larger radii. High resolution HST data are used to obtain the surface brightness profile. Together, these data sets are compared to dynamical models with varying parameters such as mass to light ratio profiles and black-hole masses.

Results. Using analytical Jeans models in combination with variable M/L_V profiles from N-body simulations we find that the best fit model is a no black hole solution. After applying various Monte Carlo simulations to estimate the uncertainties, we derive an upper limit of the back hole mass of M_BH < 1 × 10⁴ M_⊙ (with 95% confidence limits) and a global mass-to-light ratio of M/L_V = (2.1 ± 0.2) M_⊙/L_⊙.