Keck Spectroscopy of Globular Clusters around NGC 1399

We report moderate-resolution, high signal-to-noise ratio spectroscopy of globular clusters around NGC 1399, the central cD galaxy in the Fornax Cluster. We address issues as diverse as elemental abundances of globular clusters versus stellar populations in elliptical galaxies, blue horizontal branches in metal-rich globular clusters, broadband colors as metallicity tracers, possible overestimation of the age-metallicity degeneracy in globular clusters, and dark matter in the halo of NGC 1399. We obtained spectra for 21 globular cluster candidates with multislit spectroscopy using the Low Resolution Imaging Spectrograph on the Keck I Telescope. Our sample turned out to include 18 globular clusters, one star, and two low-redshift late-type galaxies (z ≃ 0.3). The mean velocity of our globular cluster sample is 1293 ± 71 km s-1, and its velocity dispersion is 302 ± 51 km s-1. Both are slightly lower than, but in agreement with, previously derived values. We derive a mass of (1–5) × 1012 M☉ within 28 kpc for the galaxy, and an M/LB ratio of 36 ± 20 or 76 ± 40 M☉/L☉B, depending on the mass estimator. Both estimates indicate that dark matter dominates the potential at 6reff. The derived elemental abundances for the globular clusters span the entire range observed in the Milky Way and M31, with a mean metallicity of our sample around [Fe/H] ≃ -0.8 dex. This implies that the two major subpopulations reported from photometry could have formed by the same processes that formed halo and disk/bulge globular clusters in the Local Group spiral galaxies. Two globular clusters, which we associate with a group of very red globular clusters, representing about 5% of the total system, clearly stand out and exhibit metal abundances as high as those observed for stellar populations in giant elliptical galaxies. In addition, they display surprisingly high Hβ and Hγ indices that are not explained by any age/metallicity combination of existing models. The high Mg and Hβ values in these clusters could, however, be explained by the presence of blue horizontal branches. Finally, we find that V - I and metallicity are well correlated in the globular cluster system, but also that the slope of the relation is twice as flat at high metallicities as an extrapolation from the relation for Milky Way globular clusters. This implies that the mean metallicities of globular cluster systems in elliptical galaxies are lower, and cover a smaller range, than ones previously derived from broadband V - I colors.