2013A&A...554A..63F

Intermediate-mass black holes (IMBHs) fill the gap between stellar-mass black holes and supermassive black holes (SMBHs). The existence of the latter is widely accepted, but there are only few detections of intermediate-mass black holes (10²-10⁵M_☉) so far. Simulations have shown that intermediate-mass black holes may form in dense star clusters, and therefore may still be present in these smaller stellar systems. Also, extrapolating the M_o-σ scaling relation to lower masses predicts intermediate-mass black holes in systems with σ∼10-20km/s such as globular clusters. We investigate the Galactic globular cluster NGC 5286 for indications of a central intermediate-mass black hole using spectroscopic data from VLT/FLAMES, velocity measurements from the Rutgers Fabry Perot at CTIO, and photometric data from HST/ACS. We compute the photometric center, a surface brightness profile, and a velocity-dispersion profile. We run analytic spherical and axisymmetric Jeans models with different central black-hole masses, anisotropy, mass-to-light ratio, and inclination. Further, we compare the data to a grid of N-body simulations without tidal field. Additionally, we use one N-body simulation to check the results of the spherical Jeans models for the total cluster mass. Both the Jeans models and the N-body simulations favor the presence of a central black hole in NGC 5286 and our detection is at the 1- to 1.5-σ level. From the spherical Jeans models we obtain a best fit with black-hole mass M_o=(1.5±1.0)x10³M_☉. The error is the 68% confidence limit from Monte Carlo simulations. Axisymmetric models give a consistent result. The best fitting N-body model is found with a black hole of 0.9% of the total cluster mass (4.38±0.18)x10⁵M_☉, which results in an IMBH mass of M_o=(3.9±2.0)x10³M_☉. Jeans models give values for the total cluster mass that are lower by up to 34% due to a lower value of M/L. Our test of the Jeans models with N-body simulation data shows that the discrepancy in the total cluster mass has two reasons: The influence of a radially varying M/L profile, and underestimation of the velocity dispersion as the measurements are limited to bright stars, which have lower velocities than fainter stars. We conclude that detection of IMBHs in Galactic globular clusters remains a challenging task unless their mass fractions are above those found for SMBHs in nearby galaxies.