M_• − σrelation for intermediate-mass black holes in globular clusters

¹ European Southern Observatory (ESO), Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
e-mail: nluetzge@eso.org
² Gemini Observatory, Northern Operations Center, 670 N. A’ohoku Place Hilo, Hawaii, 96720, USA
³ School of Mathematics and Physics, University of Queensland, Brisbane, QLD 4072, Australia
⁴ Instituto de Astronomia, Universidad Nacional Autonoma de Mexico (UNAM), A.P. 70-264, 04510 Mexico, Mexico
⁵ University Observatory, Ludwig Maximilians University, 81679 Munich, Germany
⁶ Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
⁷ Astronomy Department, University of Texas at Austin, Austin, TX 78712, USA
⁸ I.Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany

Received: 29 January 2013
Accepted: 26 April 2013

Abstract

Context. For galaxies hosting supermassive black holes (SMBHs), it has been observed that the mass of the central black hole (M_•) tightly correlates with the effective or central velocity dispersion (σ) of the host galaxy. The origin of this M_• − σ scaling relation is assumed to lie in the merging history of the galaxies, but many open questions about its origin and the behavior in different mass ranges still need to be addressed.

Aims. The goal of this work is to study the black-hole scaling relations for low black-hole masses, where the regime of intermediate-mass black holes (IMBHs) in globular clusters (GCs) is entered.

Methods. We collected all existing reports of dynamical black-hole measurements in GCs, providing black-hole masses or upper limits for 14 candidates. We plotted the black-hole masses versus different cluster parameters including total mass, velocity dispersion, concentration, and half-mass radius. We searched for trends and tested the correlations to quantify their significance using a set of different statistical approaches. For correlations with a high significance we performed a linear fit, accounting for uncertainties and upper limits.

Results. We find a clear correlation between the mass of the IMBH and the velocity dispersion of the GC. As expected, the total mass of the GC then also correlates with the mass of the IMBH. While the slope of the M_• − σ correlation differs strongly from the one observed for SMBHs, the other scaling relations M_• − M_tot, and M_• − L are similar to the correlations in galaxies. Significant correlations of black-hole mass with other cluster properties were not found in the present sample.