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G. Meylan (STScI/ESA), A. Sarajedini (Wesleyan), P. Jablonka (Paris-Meudon), S.G. Djorgovski (Caltech), T. Bridges (AAO), R.M. Rich (UCLA)
Our HST/WFPC2 observations of G1, the brightest globular cluster belonging to M31, provide CMDs which reach stars fainter than V = 27 mag, with a well populated red horizontal branch at about V = 25.3 mag. From model fitting, we determine a rather high mean metallicity of [Fe/H] = --0.95, somewhat similar to 47 Tucanae. Artificial star experiments provide us with an estimate of our measurement errors and suggest the existence of a clear color spread in the cluster red giant branch which we attribute to an intrinsic metallicity dispersion among the stars of G1; this may be the consequence of self enrichment during the early stellar/dynamical evolutionary phases of this cluster. So far, only w Centauri, the giant Galactic globular cluster, has been known to exhibit such an intrinsic metallicity dispersion.
Three mass estimates (King, Virial, and King-Michie), using HST/WFPC2 surface brightness profile and KECK/HIRES central velocity dispersion, provide values between 7 and 17 million solar masses, indicating that G1 is more than three times as massive as w Centauri, the most massive Galactic globular cluster.
Such large masses, with their deep potential wells, relate to the metallicity spreads whose origin are still unknown (either self-enrichement, or inhomogeneous proto-cluster cloud, or remaining core of a dwarf galaxy). Both clusters appear on the sequences (Kormendy 1985) defined by globular clusters, and definitely away from the other sequences defined by elliptical galaxies, bulges, and dwarf spheroidal galaxies. Nothing is known about the positions, in these diagrams, of the nuclei of nucleated dwarf elliptical galaxies. At the moment, only the correlation of metallicity with age observed in w Centauri suggests that this cluster enriched itself over a timescale of about 3 Gyr, favoring its origin as being the remaining core of a former nucleated dwarf elliptical galaxy. Massive globular clusters may simply blur the formerly clear (or simplistic) difference between globular clusters and dward galaxies.
The author(s) of this abstract have provided an email address for comments about the abstract: gmeylan@stsci.edu