Indication for an intermediate-mass black hole in the globular cluster NGC 5286 from kinematics

Abstract

Context. 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 (10 2 −10 5 M ) 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 • -σ scaling relation to lower masses predicts intermediatemass black holes in systems with σ ∼ 10−20 km s −1 such as globular clusters. Aims. 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. Methods. 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. Results. 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 • = (1.5 ± 1.0) × 10 3 M . 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) × 10 5 M , which results in an IMBH mass of M • = (3.9 ± 2.0) × 10 3 M . 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.

N-body modelling of globular clusters: masses, mass-to-light ratios and intermediate-mass black holes

“…However, since the best-fitting no-IMBH model has a reduced χ 2 value near one, the IMBH detection is not significant. This confirms the results of Feldmeier et al (2013). In NGC 6388 the best-fitting no-IMBH model fits the surface density profile better than the best-fitting IMBH model.…”

On central black holes in ultra-compact dwarf galaxies

“…Gehbardt et al 2002;Ulvestad et al 2007;Maccarone et al 2007Maccarone et al , 2008Noyola et al 2010;Anderson & van der Marel 2010;Cseh et al 2010;Lützgendorf et al 2013;Feldmeier et al 2013;Lanzoni et al 2013). For any of the above studies, the upper mass limits for central BHs do not exceed a few percent of the total cluster mass.…”

N-body simulations of globular clusters in tidal fields: Effects of intermediate-mass black holes

“…To use realistic values for the IMBH masses we considered several scaling relations such as the M • − M bulge relation (Magorrian et al 1998;Häring & Rix 2004) and the M • − σ relation (Ferrarese & Merritt 2000;Gebhardt et al 2000;Gültekin et al 2009). When these empirical correlations are extrapolated, they predict IMBH masses of M • ∼ 0.001−0.03 M tot in GCs (Goswami et al 2012).…”