  Color distributions of globular clusters (GCs) in most massive galaxies are
bimodal. Assuming linear color-to-metallicity conversions, bimodality is viewed
as the presence of merely two GC subsystems with distinct metallicities, which
serves as a critical backbone of various galaxy formation theories. Recent
studies, however, revealed that the color-metallicity relations (CMRs) often
used to derive GC metallicities (e.g., CMRs of g-z, V-I and C-T1) are in fact
inflected. Such inflection can create bimodal color distributions if the
underlying GC metallicity spread is simply broad as expected from the
hierarchical merging paradigm of galaxy formation. In order to test the
nonlinear-CMR scenario for GC color bimodality, the u-band photometry is
proposed because the u-related CMRs (e.g., CMRs of u-g and u-z) are
theoretically predicted to be least inflected and most distinctive among
commonly used optical CMRs. Here, we present Hubble Space Telescope (HST)/WFC3
F336W (u-band) photometry of the GC system in M84, a giant elliptical in the
Virgo galaxy cluster. Combining the u data with the existing HST ACS/WFC g and
z data, we find that the u-z and u-g color distributions are different from the
g-z distribution in a very systematic manner and remarkably consistent with our
model predictions based on the nonlinear-CMR hypothesis. The results lend
further confidence to validity of the nonlinear-CMR scenario as an explanation
for GC color bimodality. There are some GC systems showing bimodal
spectroscopic metallicity, and in such systems the inflected CMRs often create
stronger bimodality in the color domain.
