Metallicity distributions (MDs) of globular clusters (GCs) provide crucial
clues for the assembly and star formation history of their host galaxies. GC
colors, when GCs are old, have been used as a proxy of GC metallicities.
Bimodal GC color distributions (CDs) observed in most early-type galaxies have
been interpreted as bimodal MDs for decades, suggesting the presence of merely
two GC subpopulations within single galaxies. However, the conventional view
has been challenged by a new theory that nonlinear metallicity-to-color
conversion can cause bimodal CDs from unimodal MDs. The unimodal MDs seem
natural given that MDs involved many thousand protogalaxies. The new theory has
been tested and corroborated by various observational and theoretical studies.
Here we examine the nonlinear nature of GC color-metallicity relations (CMRs)
using photometric and spectroscopic GC data of NGC 5128 (Centaurus A) and NGC
4594 (Sombrero), in comparison with stellar population simulations. We find
that, with a slight offset in color, the overall shapes of observed and modeled
CMRs agree well for all available colors. Diverse color-depending morphologies
of GC CDs of the two galaxies are well reproduced based on their observed
spectroscopic MDs via our CMR models. The results corroborate the nonlinear CMR
interpretation of the GC color bimodality, shedding further light on theories
of galaxy formation.