  Due to its proximity (9 Mpc) and the strongly bimodal color distribution of
its spectroscopically well-sampled globular cluster (GC) system, the early-type
galaxy NGC 3115 provides one of the best available tests of whether the color
bimodality widely observed in GC systems generally reflects a true metallicity
bimodality. Color bimodality has alternatively been attributed to a strongly
nonlinear color--metallicity relation reflecting the influence of hot
horizontal branch stars. Here we couple Subaru Suprime-Cam gi photometry with
Keck/DEIMOS spectroscopy to accurately measure GC colors and a CaT index that
measures the CaII triplet. We find the NGC 3115 GC system to be unambiguously
bimodal in both color and the CaT index. Using simple stellar population
models, we show that the CaT index is essentially unaffected by variations in
horizontal branch morphology over the range of metallicities relevant to GC
systems (and is thus a robust indicator of metallicity) and confirm bimodality
in the metallicity distribution. We assess the existing evidence for and
against multiple metallicity subpopulations in early and late-type galaxies and
conclude that metallicity bi/multimodality is common. We briefly discuss how
this fundamental characteristic links directly to the star formation and
assembly histories of galaxies.
