Ten Years of Debate on the Origin of Globular Cluster Color Bimodality

The globular cluster (GC) systems in most elliptical galaxies show bimodal color distributions. This phenomenon has been generally regarded as a bimodal metallicity distribution, indicating the presence of two sub-populations in a GC system. However, since a new explanation on the bimodality was introduced where the nonlinear metallicity-to-color conversion can cause bimodal color distributions, the origin of this phenomenon has been under hot debate. In this presentation, we briefly review the ten-year debate on the origin of GC color bimodality, and present our recent pieces of evidence on the nonlinear nature of GC color-metallicity relations.