Mayall II=G1 in M31: Giant Globular Cluster or Core of a Dwarf Elliptical Galaxy?

Mayall II=G1 is one of the brightest globular clusters belonging to M31, the Andromeda galaxy. Our observations with the Wide Field and Planetary Camera (WFPC2) on board the Hubble Space Telescope (HST) provide photometric data for the I versus V-I and V versus V-I color-magnitude diagrams. They reach stars with magnitudes 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+/-0.09, somewhat similar to 47 Tucanae. In order to determine our true measurement errors, we have carried out artificial star experiments. We find a larger spread in V-I than can be explained by the measurement errors, and we attribute this to an intrinsic metallicity dispersion amongst 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 ω Centauri, the giant Galactic globular cluster, has been known to exhibit such an intrinsic metallicity dispersion, a phenomenon certainly related to the deep potential wells of these two star clusters. We determine, from the same HST/WFPC2 data, the structural parameters of G1. Its surface brightness profile provides its core radius r_c=0.14"=0.52 pc, its tidal radius r_t~=54^''=200 pc, and its concentration c=log(r_t/r_c)~=2.5. Such a high concentration indicates the probable collapse of the core of G1. KECK/HIRES observations provide the central velocity dispersion σ_obs=25.1 km s^-1, with σ_p(0)=27.8 km s^-1 once aperture corrected. Three estimates of the total mass of this globular cluster can be obtained. The King-model mass is M_K=15×10⁶ M_solar with M/L_V~=7.5, and the virial mass is M_Vir=7.3×10⁶ M_solar with M/L_V~=3.6. By using a King-Michie model fitted simultaneously to the surface brightness profile and the central velocity dispersion value, mass estimates range from M_KM=14×10⁶ M_solar to 17×10⁶ M_solar. Although uncertain, all of these mass estimates make G1 more than twice as massive as ω Centauri, the most massive Galactic globular cluster, whose mass is also uncertain by about a factor of 2. G1 is not unique in M31: at least three other bright globular clusters of this galaxy have velocity dispersions σ_obs larger than 20 km s^-1, implying probably similar large masses. Such large masses relate to the metallicity spread whose origin is still unknown (either self-enrichment, an inhomogeneous proto-cluster cloud, or remaining core of a dwarf galaxy). Let us consider for G1 the four following parameters: central surface brightness μ(0, V)=13.47 mag arcsec^-2, core radius r_c=0.52 pc, integrated absolute visual magnitude M_V=-10.94 mag, and central velocity dispersion σ(0)=28 km s^-1. When considering the positions of G1 in the different diagrams defined by Kormendy using the above four parameters, G1 always appears on the sequence defined by globular clusters, and definitely away from the other sequences defined by elliptical galaxies, bulges, and dwarf spheroidal galaxies. The same is true for ω Centauri. Little is known about the positions, in these diagrams, of the nuclei of nucleated dwarf elliptical galaxies, which could be the progenitors of some of the most massive globular clusters. The above four parameters are known only for the nucleus of one dwarf elliptical, viz., NGC 205, and put this object, in the Kormendy's diagram, close to G1, right on the sequence of globular clusters. This does not prove that all (massive) globular clusters are the remnant cores of nucleated dwarf galaxies. 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. 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.