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Recent spectroscopy on the globular cluster (GC) system of M31 withunprecedented precision witnessed a clear bimodality in absorption-line indexdistributions of old GCs. Such division of extragalactic GCs, so far assertedmainly by photometric color bimodality, has been viewed as the presence ofmerely two distinct metallicity subgroups within individual galaxies and formsa critical backbone of various galaxy formation theories. Given thatspectroscopy is a more detailed probe into stellar population than photometry,the discovery of index bimodality may point to the very existence of dual GCpopulations. However, here we show that the observed spectroscopic dichotomy ofM31 GCs emerges due to the nonlinear nature of metallicity-to-index conversionand thus one does not necessarily have to invoke two separate GC subsystems. Wetake this as a close analogy to the recent view that metallicity-colornonlinearity is primarily responsible for observed GC color bimodality. We alsodemonstrate that the metallicity-sensitive magnesium line displaysnon-negligible metallicity-index nonlinearity and Balmer lines show ratherstrong nonlinearity. This gives rise to bimodal index distributions, which areroutinely interpreted as bimodal metallicity distributions, not consideringmetallicity-index nonlinearity. Our findings give a new insight into theconstitution of M31's GC system, which could change much of the current thoughton the formation of GC systems and their host galaxies.

astrophysical journal/the astrophysical journal

Nonlinear Color-Metallicity Relations of Globular Clusters. V. Nonlinear  Absorption-line Index versus Metallicity Relations and Bimodal Index  Distributions of M31 Globular Clusters