A New Explanation of Globular Cluster Color Distributions

The colors of globular clusters (GCs) in most large early-type galaxies are bimodal. This is generally taken as evidence for the presence of two GC subpopulations that have different geneses. However we show that, because of an inflection along the metallicity-color relation, a coeval group of old (>10 Gyr) GCs can exhibit bimodal color distributions even when the intrinsic metallicity distribution is unimodal. The inflection is a result of two complementary effects: (i) the integrated color of main-sequence and giant-branch is a mild non-linear function of metallicity, and (ii) the rapid change in color due to the onset of the hot horizontal-branch further strengthens the non-linearity. We demonstrate that the hypothesis gives remarkably simple and cohesive explanations for all the key observations, including the close link of the GC color distributions to the host galaxy properties.