The HST Large Programme on omega Centauri. III. Absolute Proper Motion

In this paper, we report a new estimate of the absolute proper motion (PM) of the globular cluster NGC 5139 (omega Cen) as part of the HST large program GO-14118+ 14662. We analyzed a field 17 arcmin southwest of the center of omega Cen and computed PMs with epoch spans of similar to 15.1 years. We employed 45 background galaxies to link our relative PMs to an absolute reference-frame system. The absolute PM of the cluster in our field is (mu(alpha) cos delta, mu(delta))=(-3.341. 0.028, -6.557 +/- 0.043) mas yr(-1). Upon correction for the effects of viewing perspective and the known cluster rotation, this implies that for the cluster center of mass (mu(alpha) cos delta, mu(delta))=(-3.238. 0.028, -6.716 +/- 0.043) mas yr(-1). This measurement is direct and independent, has the highest random and systematic accuracy to date, and will provide an external verification for the upcoming Gaia Data Release 2. It also differs from most reported PMs for omega Cen in the literature by more than 5 sigma, but consistency checks compared to other recent catalogs yield excellent agreement. We computed the corresponding Galactocentric velocity, calculated the implied orbit of omega Cen in two different Galactic potentials, and compared these orbits to the orbits implied by one of the PM measurements available in the literature. We find a larger (by about 500 pc) perigalactic distance for omega Cen with our new PM measurement, suggesting a larger survival expectancy for the cluster in the Galaxy.