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 omega 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) similar or equal to 52 = 200 pc, and its concentration c = log (r(t)/r(c)) similar or equal to 2.5. Such a high concentration indicates the probable collapse of the core of G1. KECK/HIRES observations provide the central velocity dispersion sigma (obs) = 25.1 km s(-1), with sigma (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 with and the virial mass is with By M-K = 15 x 10(6) M. with M/L-V similar or equal to 7.5, and the virial mass is M-Vir = 7.3 x 10(6) M. with M/L-V similar or equal to 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 x 10(6) M. to 17 x 10(6) M. Although uncertain, all of these mass estimates make G1 more than twice as massive as omega 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 sigma (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 mu (0, V) = 13.47 mag arcsec(-2), core radius r(c) = 0.52 pc, integrated absolute visual magnitude mag, and central velocity dispersion sigma (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 omega Centauri.