A New Look at Globular Cluster Colors in NGC 3311 and the Case for Exclusively Metal-rich Globular Cluster Systems

NGC 3311, the central cD galaxy in the Hydra Cluster, was previously thought to host the most metal-rich globular cluster system known. Ground-based Washington photometry had indicated the almost complete absence of the population of globular clusters near [Fe/H] ~ -1 dex; these globular clusters are normally dominant in the metallicity distribution functions of giant elliptical galaxies. Lacking the normal metal-poor globular cluster population, NGC 3311 was an outstanding exception among galaxies and was not easily understood under any of the current globular cluster formation scenarios. Our Hubble Space Telescope Wide Field Planetary Camera 2 data yield normal globular cluster colors and hence metallicities for this galaxy. We find a bimodal color distribution with peaks at (V-I)0 = 0.91 ± 0.03 and 1.09 ± 0.03, corresponding to [Fe/H] ~ -1.5 and -0.75 dex (somewhat dependent on the choice of the conversion relation between color and metallicity). We review the evidence for exclusively metal-rich globular cluster systems in other galaxies and briefly discuss the implications for our understanding of globular cluster and galaxy formation.