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

TL;DR

This study uses detailed stellar motion data in 47 Tucanae's core to test for an intermediate-mass black hole, finding that stellar remnants and binaries explain the observations without requiring an IMBH.

Contribution

It introduces a comprehensive velocity dispersion model incorporating stellar remnants and binaries, challenging previous IMBH claims in 47 Tucanae.

Findings

No evidence for an IMBH in 47 Tucanae based on velocity dispersion.

Stellar remnants and binaries account for observed core dynamics.

High retention of stellar-mass black holes is incompatible with data.

Abstract

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 exceedingly clear view of stellar motions into the very center of the crowded core, yielding proper motions for $>$50,000 stars in the central 2'. 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 templates. 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 unbinned 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. The concentrated binaries and stellar-mass black holes (BHs) produce a sufficient velocity dispersion signal in the core so as to make an IMBH unnecessary to fit the observations. We additionally determine that a stellar-mass BH retention fraction of $\gtrsim 8.5\%$ becomes incompatible with our observed velocities in the core.