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Session 98 - Globular Clusters.
Oral session, Friday, January 09

[98.04] Mayall II \equiv G1: A Giant Globular Cluster in M31

G. Meylan (ESO), P. Jablonka (Paris/Meudon), S. G. Djorgovski (Caltech), A. Sarajedini (SFSU), T. Bridges (RGO), R. M. Rich (Columbia)

Two observational constraints exist for G1: the HST/WFPC2 surface brightness profile providing the core radius r_c\ = 0.24^\prime\prime\ = 0.9 pc, the tidal radius r_t\ \simeq 54^\prime\prime\ = 200 pc, and the concentration c = log (r_t/r_c)\ \simeq 2.35; the KECK/HIRES central velocity dispersion \sigma_obs\ = 25.1 km\thinspace s^-1, with \sigma_p(0)\ = 27.3 km\thinspace s^-1\ once aperture corrected.

Two simple estimates of the total mass of this GC can be obtain. First, since King mass = \rho_c r_c^3\mu = 167 r_c \mu\sigma_p(0)^2, the total King-model mass M = 14 \times 10^6 M_ødot\ with M/L \simeq 10. Second, since Virial mass = 670 r_h \sigma_p(0)^2, the total Virial mass M = 6.8 \times 10^6 M_ødot\ with M/L \simeq 4.9.

By using a King-Michie model fitted simultaneously to the surface brightness profile and the central velocity dispersion value, and recovering the total integrated absolute luminosity M_V = --10.55 mag, a grid of about 10,000 models has been calculated for a wide range of values of each parameter. Total mass estimates range from 10 \times 10^6 M_ødot\ to 18 \times 10^6 M_ødot. The lack of a velocity dispersion profile prevents the process of narrowing down the number (< 100) of successful models, although these models populate only very specific and small areas of the parameter space.

We reach the following conclusions: \bullet All mass estimates give G1 more than twice as massive as ømega\thinspace Centauri, the most massive galactic GC. \bullet With c = log (r_t/r_c)\ = 2.35, G1 is significantly more concentrated than ømega\thinspace Cen, which has c = 1.24, and 47\thinspace Tuc, another massive galactic GC, which has c = 2.04. \bullet Although G1 is the heaviest of the weighted GCs, it would be a hasty conclusion to claim that G1, even more than ømega\thinspace Cen, is a kind of transition step between GCs and dwarf elliptical galaxies. When considering the positions of G1 in the different diagrams defined by Kormendy (1985) (envolving the parameters \mu, r_c, M_V, and \sigma_p(0)), G1 appears always close to the sequence defined by GCs, and away from the sequences defined by elliptical galaxies, bulges, and dwarf spheroidal galaxies. Consequently, although being very bright and very massive, Mayall II \equiv G1 is a genuine globular cluster.

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