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 Ca mera (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 r each stars with magnitudes fainter than V = 27 mag, with a well populated r ed 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. W e find a larger spread in V-I than can be explained by the measurement erro rs, and we attribute this to an intrinsic metallicity dispersion amongst th e stars of G1; this may be the consequence of self-enrichment during the ea rly 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 cer tainly 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 conc entration c = log (r(t)/r(c)) similar or equal to 2.5. Such a high concentr ation indicates the probable collapse of the core of G1. KECK/HIRES observa tions 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 mode l fitted simultaneously to the surface brightness profile and the central v elocity 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, whos e mass is also uncertain by about a factor of 2. G1 is not unique in M31: a t least three other bright globular clusters of this galaxy have velocity d ispersions sigma (obs) larger than 20 km s(-1), implying probably similar l arge masses. Such large masses relate to the metallicity spread whose origin is still un known (either self-enrichment, an inhomogeneous proto-cluster cloud, or rem aining core of a dwarf galaxy). Let us consider for G1 the four following p arameters: central surface brightness mu (0, V) = 13.47 mag arcsec(-2), cor e radius r(c) = 0.52 pc, integrated absolute visual magnitude mag, and cent ral velocity dispersion sigma (0) = 28 km s(-1). When considering the posit ions of G1 in the different diagrams defined by Kormendy using the above fo ur parameters, G1 always appears on the sequence defined by globular cluste rs, and definitely away from the other sequences defined by elliptical gala xies, bulges, and dwarf spheroidal galaxies. The same is true for omega Cen tauri.