Placing the Fornax and Sagittarius Dwarf Spheroidal Globular Clusters in the Horizontal-Branch Type versus Metallicity Diagram

Abstract

We plot the globular clusters of the Fornax galaxy and those associated with the Sagittarius dwarf spheroidal galaxy in the horizontal-branch type versus metallicity diagram. The horizontal-branch types for the Fornax clusters include corrections for red horizontal-branch stars from the field and are based on our recent work and new results in the literature. Fornax globular clusters continue to stand out as having red horizontal branches for their low ([Fe/H] ~ -2) metallicities, with no counterparts in either the outer Galactic halo or the Magellanic Clouds. The clusters associated with Sagittarius lie to the blue of the Fornax clusters, except for the metal-rich cluster Ter 7. Although the metallicities of the three metal-poor Sagittarius globular clusters are similar to those of the Fornax clusters, their horizontal branches are bluer and they lie in a region also populated by the old LMC and old halo clusters. Neither cluster system resembles the younger Galactic halo globular clusters, often suggested to have been accreted from disrupted dwarf spheroidal galaxies. Except for Ter 7, both the Fornax and Sagittarius globular clusters are metal-poor compared with their Galactic counterparts of the same horizontal-branch type. We find no correlation between HB type and other cluster properties such as central concentration, luminosity, central surface brightness, and estimated collision rate.