Kinematic complexity and black holes in dense star clusters

Lead Research Organisation: University of Edinburgh
Department Name: Sch of Mathematics

Abstract

Globular clusters are large, dense collections of tens of thousands, or millions of stars all bound together by gravity. They are also one of the main candidates as hosts of intermediate mass black holes (IMBH). As the name suggests, these are black holes with masses higher than those formed from the collapse of individual stars, called stellar mass black holes, which have masses in the 10-100 solar mass range , but significantly less than the supermassive black holes which are typically found in the centres of galaxies and can have masses in the millions, or billions of solar masses.

Until recently there was no direct observational evidence of the existence of an IMBH. This general lack of direct observational evidence has made indirect approaches, using the dynamics of the stars within a globular cluster, an important tool in the search for IMBHs. In these approaches we use global properties of the cluster (density, velocity dispersion etc.) to infer the presence of a black hole, rather than "seeing" it. This relies on having accurate models of a globular cluster, which is where my work is currently focused.

Traditionally, globular clusters have been well modelled under the assumptions of spherical symmetry, and lack of internal rotation. However, recent high quality data, especially from the hubbble space telescope and GAIA, has allowed the determination of the internal kinematics of many globular clusters, in detail, for the first time. This has revealed that many globular cluster exhibit both significant degrees of internal rotation, and anisotropy in velocity space. These are effects that haven't been taken account of in much of the literature examining the presence of IMBHs in globular clusters.

My research is looking to address this by adapting a method for the inclusion of a central black hole at the centre of a galaxy, to the context of a globular cluster. Essentially we take a commonly used globular cluster model, the King model, and use the equation of hydrostatic equilibrium to derive a modifed boundary condition that takes account of the central black hole. Initially we will examine the spherical, non-rotating case as a test bed for the method, looking first to explain the rapid transitional behaviour between two distinct physical regimes in this case. This behaviour has been noted by previous authors but a full characterisation is still needed. We will then move on to apply the same procedure to include the influence of a central black hole within a model for a rigidly rotating cluster that was developed by my supervisor. This will provide a new class of equilibrium models for a rotating globular cluster with a central black hole.

Once these models have been constructed we will then be in a position to examine a range of interesting questions. For example, examining how the IMBH affects the properties of the cluster, which will potentially aid in the indirect search for IMBHs within globular clusters. With the advent of gravitational wave astronomy there are also questions to be answered about how the combination of a black hole and kinematic complexity may affect the rate at which black hole mergers are seen, for example.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/S023291/1 01/10/2019 31/03/2028
2277589 Studentship EP/S023291/1 01/09/2019 31/08/2023 Samuel Richard Bonsor