HST's hunt for intermediate-mass black holes in star clusters

Abstract

Abstract. Establishing or ruling out, either through solid mass measurements or upper limits, the presence of intermediate-mass black holes (IMBHs; with masses of 10 2 − 10 5 M ) at the centers of star clusters would profoundly impact our understanding of problems ranging from the formation and long-term dynamical evolution of stellar systems, to the nature of the seeds and the growth mechanisms of supermassive black holes. While there are sound theoretical arguments both for and against their presence in today's clusters, observational studies have so far not yielded truly conclusive IMBH detections nor upper limits. We argue that the most promising approach to solving this issue is provided by the combination of measurements of the proper motions of stars at the centers of Galactic globular clusters and dynamical models able to take full advantage of this type of data set. We present a program based on HST observations and recently developed tools for dynamical analysis designed to do just that.

Investigating the retention of intermediate-mass black holes in star clusters usingN-body simulations

“…A direct proof would come from detailed kinematical observations of stars moving under the influence of the IMBH at the centers of GCs. Unfortunately, the radius of influence of an IMBH is only of a few arc seconds (Peebles 1972;Chanamé et al 2010;Miller & Colbert 2004), so it is very difficult, if not impossible, to accurately determine its mass by measuring the velocities of stars moving under its influence, with the currently available instruments. This technique has been successfully used for determining the mass of the SMBH at the centre of the Milky Way galaxy, where the stellar environment is less dense than the core of GCs and also there exists a number of young and bright stars, moving under the gravitational influence of the SMBH which have been followed by observations for more than 15 years (Gillessen et al 2009).…”

“…Also, since GCs are old systems, this small sphere of influence contains mainly massive stellar remnants and old, dim stars that could not be easily observed and traced. For the above reasons, kinematical techniques can currently only give upper limits on the mass of the potential IMBHs at the centers of galactic GCs (Anderson & van der Marel 2010;van der Marel & Anderson 2010;Noyola et al 2010;Lützgendorf et al 2012) (see also Kirsten & Vlemmings 2012;Strader et al 2012, for observations that do not support the IMBH scenario). Since such limits are based on measurements of proper motions, velocity dispersion or line of sight motions away of the sphere of influence of the potential IMBH, alternative to IMBH explanations cannot be ruled out (Baumgardt et al , 2005.…”

Exploring intermediate and massive black-hole binaries with the Einstein Telescope

“…Limits on the presence of IMBHs in NGC 6254 [15] and NGC 2298 [91] have been placed based on arguments related to the suppression of mass segregation by any IMBHs in those clusters [43]. It has been suggested that measurements of proper motions in globulars will be needed to rule definitively one way or another [28], but until then we are left with some uncertainty. Depending on the threshold one adopts for IMBH likelihood, candidates exist in one out of every tens to hundreds of galaxies.…”