Mayall II equiv G1: A Giant Globular Cluster in M31

Details Mayall II equiv G1: A Giant Globular Cluster in M31 Mayall II equiv G1: A Giant Globular Cluster in M31

Dec 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997aas...191.9804m&link_type=abstract

American Astronomical Society, 191st AAS Meeting, #98.04; Bulletin of the American Astronomical Society, Vol. 29, p.1367

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Two observational constraints exist for G1: the HST/WFPC2 surface brightness profile providing the core radius r_c\ = 0.24('') \ = 0.9 pc, the tidal radius r_t\ =~ 54('') \ = 200 pc, and the concentration c = log (r_t/r_c)\ =~ 2.35; the KECK/HIRES central velocity dispersion sigma_ {obs}\ = 25.1 kms(-1) , with sigma_p (0)\ = 27.3 kms(-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 x { {10(6) Msun}}\ with M/L =~ 10. Second, since Virial mass = 670 r_h sigma_p (0)(2) , the total Virial mass M = 6.8 x { {10(6) Msun}}\ with M/L =~ 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 x { {10(6) Msun}}\ to 18 x { {10(6) Msun}}. 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: All mass estimates give G1 more than twice as massive as omega Centauri, the most massive galactic GC. With c = log (r_t/r_c)\ = 2.35, G1 is significantly more concentrated than omega Cen, which has c = 1.24, and 47Tuc, another massive galactic GC, which has c = 2.04. Although G1 is the heaviest of the weighted GCs, it would be a hasty conclusion to claim that G1, even more than omega 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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