Nonlinear Color-Metallicity Relations of Globular Clusters. X.
  Subaru/FOCAS Multi-object Spectroscopy of M87 Globular Clusters
  We obtained spectra of some 140 globular clusters (GCs) associated with the
Virgo central cD galaxy M87 with the Subaru/FOCAS MOS mode. The fundamental
properties of GCs such as age, metallicity and $\alpha$-element abundance are
investigated by using simple stellar population models. It is confirmed that
the majority of M87 GCs are as old as, more metal-rich than, and more enhanced
in $\alpha$-elements than the Milky Way GCs. Our high-quality, homogeneous
dataset enables us to test the theoretical prediction of inflected
color$-$metallicity relations (CMRs). The nonlinear-CMR hypothesis entails an
alternative explanation for the widely observed GC color bimodality, in which
even a unimodal metallicity spread yields a bimodal color distribution by
virtue of nonlinear metallicity-to-color conversion. The newly derived CMRs of
old, high-signal-to-noise-ratio GCs in M87 (the $V-I$ CMR of 83 GCs and the
$M-T2$ CMR of 78 GCs) corroborate the presence of the significant inflection.
Furthermore, from a combined catalog with the previous study on M87 GC
spectroscopy, we find that a total of 185 old GCs exhibit a broad, unimodal
metallicity distribution. The results corroborate the nonlinear-CMR
interpretation of the GC color bimodality, shedding further light on theories
of galaxy formation.
