Abstract
At the moment, only the anticorrelation of metallicity with age recently observed in ω Centauri suggests that this cluster enriched itself over a timescale of about 3 Gyr. This contradicts the general idea that all the stars in a globular cluster are coeval and may favor the origin of ω Centauri as being the remaining core of a larger entity, e.g., of a former nucleated dwarf elliptical galaxy. In any case, the very massive globular clusters, by the mere fact that their large masses imply complicated stellar and dynamical evolution, may blur the former clear (or simplistic) difference between globular clusters and dwarf galaxies.
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Footnotes
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Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associate with proposal IDs 5907 and 5464.
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Based in part on observations obtained at the W. M. Keck Observatory, which is operated jointly by the California Institute of Technology and the University of California.