New Limits on an Intermediate Mass Black Hole in Omega Centauri: I. Hubble Space Telescope Photometry and Proper Motions

TL;DR

This study uses Hubble Space Telescope data to analyze the core dynamics of Omega Centauri, finding no evidence for an intermediate-mass black hole and providing detailed proper-motion measurements.

Contribution

First detailed proper-motion dispersion profile in Omega Centauri showing no signs of an IMBH, with new methods for center determination and mass-dependent kinematic analysis.

Findings

Proper-motion dispersion increases inward but shows no cusp.

No stars with unusually high velocities detected.

Proper-motion dispersion varies with stellar mass, indicating lack of equipartition.

Abstract

We analyze data from the Hubble Space Telescope's Advanced Camera for Surveys of the globular cluster Omega Cen. We construct a photometric and proper-motion catalog using the GO-9442, GO-10252, and GO-10775 data sets. The 2.5- to 4-year baseline between observations yields a catalog of some 10^5 proper motions, with 53,382 high-quality measurements in a central field. We determine the cluster center to ~1-arcsecond accuracy using two different star-count methods, and a completely independent method using 2MASS images. We also determine the kinematical center of the proper motions, which agrees with the star-count center to within its uncertainty. The proper-motion dispersion of the cluster increases gradually inwards, but there is no variation in kinematics with position within the central ~15 arcsec: there is no dispersion cusp and no stars with unusually high velocities. We measure for the first time in any globular cluster the variation in proper-motion dispersion with mass along the main sequence, and find the cluster not yet to be in equipartition. Our proper-motion results do not confirm the arguments put forward by Noyola, Gebhardt & Bergmann to suspect an intermediate-mass black hole (IMBH) in Omega Cen. In Paper II we present new dynamical models for the high-quality data presented here, with the aim of putting quantitative contraints on the mass of any possible IMBH.