UBC Theses and Dissertations
A multi-mass velocity dispersion model of 47 Tucanae : no evidence for an intermediate-mass black hole Mann, Christopher Rhys
In this thesis we analyze stellar proper motions in the core of the globular cluster 47 Tucanae to explore the possibility of an intermediate-mass black hole (IMBH) influence on the stellar dynamics. Our use of short-wavelength photometry affords us an unprecedentedly clear view of stellar motions into the very centre of the crowded core, yielding proper motions for >50,000 stars in the central 2 arcminutes. We model the velocity dispersion profile of the cluster using an isotropic Jeans model. The density distribution is taken as a central IMBH point mass added to a combination of King profiles. We individually model the general low-mass cluster objects (main sequence/giant stars), as well as the concentrated populations of heavy binary systems and dark stellar remnants. Using un-binned likelihood model-fitting, we find that the inclusion of the concentrated populations in our model plays a crucial role in fitting for an IMBH mass. Taking into account all of these cluster sub-populations our model predicts an IMBH to cluster mass ratio of 0.06% +/- 0.13%. The concentrated binaries and stellar-mass black holes produce a sufficient enhancement to the velocity dispersion signal in the core as to make an IMBH unnecessary to fit the observations. We additionally determine that a stellar-mass black hole retention fraction of >18% becomes incompatible with our kinematic observations for 47 Tuc.
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