Nonlinear Color-Metallicity Relations of Globular Clusters. V. Nonlinear
  Absorption-line Index versus Metallicity Relations and Bimodal Index
  Distributions of M31 Globular Clusters
  Recent spectroscopy on the globular cluster (GC) system of M31 with
unprecedented precision witnessed a clear bimodality in absorption-line index
distributions of old GCs. Such division of extragalactic GCs, so far asserted
mainly by photometric color bimodality, has been viewed as the presence of
merely two distinct metallicity subgroups within individual galaxies and forms
a critical backbone of various galaxy formation theories. Given that
spectroscopy is a more detailed probe into stellar population than photometry,
the discovery of index bimodality may point to the very existence of dual GC
populations. However, here we show that the observed spectroscopic dichotomy of
M31 GCs emerges due to the nonlinear nature of metallicity-to-index conversion
and thus one does not necessarily have to invoke two separate GC subsystems. We
take this as a close analogy to the recent view that metallicity-color
nonlinearity is primarily responsible for observed GC color bimodality. We also
demonstrate that the metallicity-sensitive magnesium line displays
non-negligible metallicity-index nonlinearity and Balmer lines show rather
strong nonlinearity. This gives rise to bimodal index distributions, which are
routinely interpreted as bimodal metallicity distributions, not considering
metallicity-index nonlinearity. Our findings give a new insight into the
constitution of M31's GC system, which could change much of the current thought
on the formation of GC systems and their host galaxies.
