The HST large programme on ω centauri – III. Absolute proper motion

Mattia Libralato, Andrea Bellini, Luigi R. Bedin, Edmundo D. Moreno, José G. Fernández-Trincado, Barbara Pichardo, Roeland P. van der Marel, Jay Anderson, Dániel Apai, Adam J. Burgasser, Anna Fabiola Marino, Antonino P. Milone, Jon M. Rees, Laura L. Watkins

Research output: Contribution to journalArticlepeer-review


In this paper we report a new estimate of the absolute proper motion (PM) of the globular cluster NGC 5139 (ω Cen) as part of the HST large program GO-14118+14662. We analyzed a field 17 arcmin South-West of the center of ω Cen and computed PMs with an epoch span of ∼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: (µα cos δ, µδ) = (−3.341±0.028, −6.557±0.043) mas yr1. Upon correction for the effects of viewing perspective and the known cluster rotation, this implies that for the cluster center of mass (µα cos δ, µδ) = (−3.238 ± 0.028, −6.716 ± 0.043) mas yr1. 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 ω Cen in the literature by more than 5σ, but consistency checks compared to other recent catalogs yield excellent agreement. We computed the corresponding Galactocentric velocity, calculated the implied orbit of ω 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 ω Cen with our new PM measurement, suggesting a larger survival expectancy for the cluster in the Galaxy.

Original languageEnglish (US)
JournalUnknown Journal
StatePublished - Jan 4 2018


  • Astrometry
  • Dynamics
  • Galaxy: kinematics
  • Globular clusters: individual (NGC 5139 (ω Cen))
  • Proper motions

ASJC Scopus subject areas

  • General

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