European Southern Observatory (ESO),
2 Max-Planck-Institut für Extraterrestrische Physik, 85748 Garching, Germany
3 University Observatory, Ludwig Maximilians University, Munich 81679, Germany
4 Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
5 Astronomy Department, University of Texas at Austin, Austin, TX 78712, USA
6 School of Mathematics and Physics, University of Queensland, Brisbane, QLD 4072, Australia
Received: 1 February 2011
Accepted: 20 July 2011
Context. Intermediate-mass black holes (IMBHs) are of interest in a wide range of astrophysical fields. In particular, the possibility of finding them at the centers of globular clusters has recently drawn attention. IMBHs became detectable since the quality of observational data sets, particularly those obtained with HST and with high resolution ground based spectrographs, advanced to the point where it is possible to measure velocity dispersions at a spatial resolution comparable to the size of the gravitational sphere of influence for plausible IMBH masses.
Aims. We present results from ground based VLT/FLAMES spectroscopy in combination with HST data for the globular cluster NGC 6388. The aim of this work is to probe whether this massive cluster hosts an intermediate-mass black hole at its center and to compare the results with the expected value predicted by the M• − σ scaling relation.
Methods. The spectroscopic data, containing integral field unit measurements, provide kinematic signatures in the center of the cluster while the photometric data give information of the stellar density. Together, these data sets are compared to dynamical models and present evidence of an additional compact dark mass at the center: a black hole.
Results. Using analytical Jeans models in combination with various Monte Carlo simulations to estimate the errors, we derive (with 68% confidence limits) a best fit black-hole mass of (17 ± 9) × 103 M⊙ and a global mass-to-light ratio of M/LV = (1.6 ± 0.3) M⊙/L⊙.
Key words: black hole physics / globular clusters: individual: NGC 6388 / stars: kinematics and dynamics
Based on observations collected at the European Organization for Astronomical Research in the Southern Hemisphere, Chile (083.D-0444).
HST photometry (data for Fig. 1) and ARGUS velocity map (data for Fig. 7) are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (22.214.171.124) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/533/A36
© ESO, 2011