Abundances and Kinematics of the Globular Cluster Systems of the Galaxy and of the Sagittarius Dwarf

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Abstract

Spectra have been obtained at the Ca II triplet of a number of member giants in seven galactic globular clusters for which current metallicity estimates are either lacking or poorly determined. Similar spectra have also been obtained of giants in eight clusters whose abundances are well established. These latter data have been combined with other existing CaII triplet observations to define a (Ca II triplet line strength, abundance) relation that encompasses the full range of metallicities exhibited by the Galaxy's globular cluster system. This calibration is then used to determine abundances for the program clusters. Specifically, we find [Fe/Il] -2.00+/-0.08, -0.67+/-0.07 -128+/-0.12, -0.36+/-0.09, -1.70+/-0.11, -1.55+/-0.10, and -1.99+/-0.08 for Pal 15, NGC 6366, NGC 6626 (M28), Terzan 7, Asp 2, NGC 6715 (M54), and Terzan 8, respectively. Further, although our sample in M54 is small (five stars), our results are consistent with the color-magnitude diagram based findings of Sarajedini & Layden [AJ, 109,1086 (1995)], who have suggested that M54 may possess an internal abundance range characterized by σ([Fe/H])~0.15 dex. Radial velocities for the program clusters have also been derived from our spectra. With these new abundance and velocity data, and using other recent results from the literature, we have repeated the analysis of Zinn [ASP ConE. Ser. Vol. 48, p. 38 (1993)]. We confirm his finding of kinematic differences between two subsamples of the halo globular clusters. In particular, we find for the "younger halo" clusters, which have redder horizontal branch morphologies and/or younger ages at fixed abundance, a velocity dispersion σ_los_ = 163+/-25 km/s and a (retrograde) rotation of V_rot_ = -46+/-81 km/s. For the "old halo" clusters, which have bluer horizontal branch morphologies, the corresponding numbers are σ_los_= 106+/-11 km/s and V_rot_ 50+/-27 km/s. We note also that four of the clusters studied here (M54, Terzan 7, Arp 2, and Terzan 8) locations in the sky, radial velocities, and distances that associate them with the recently discovered Sagittarius dwarf spheroidal galaxy. We then compare the properties of this presumed Sagittarius cluster system with the cluster system of the Fornax dwarf spheroidal galaxy, noting in particular that for both systems the mean cluster abundance is considerably less than the mean abundance of the field star population. We note also that the Sagittarius cluster system, if integrated into the galactic halo, will contribute clusters to both the younger halo and old halo subsamples.

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