A Multi-Mass Velocity Dispersion Model of 47 Tucanae Indicates No Evidence for an Intermediate Mass Black Hole

Christopher Mann, Harvey Richer, Jeremy Heyl, Jay Anderson, Jason Kalirai, Ilaria Caiazzo, Swantje Möhle, Alan Knee, Holger Baumgardt

Published 2018-07-09Version 1

In this paper 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 $0.06\% \pm 0.13\%$. The concentrated binaries and stellar-mass black holes produce a sufficient 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 $\gtrsim 18\%$ becomes incompatible with our observations for 47 Tuc.