Nonlinear Color–Metallicity Relations of Globular Clusters. VIII. Reproducing Color Distributions of Individual Globular Cluster Systems in the Virgo and Fornax Galaxy Clusters

Abstract

The color distributions of globular clusters (GCs) in individual early-type galaxies show great diversity in their morphology. Based on the conventional "linear" relationship between the colors and metallicities of GCs, the GC metallicity distributions inferred from colors and in turn their formation histories, should be as diverse as they appear. In contrast, here we show that an alternative scenario rooted in the "nonlinear" nature of the metallicity-to-color transformation points to a simpler and more coherent picture. Our simulations of the color distributions for $\sim$80 GC systems in early-type galaxies from the ACS Virgo and Fornax Cluster Surveys suggest that the majority ($\sim$70$\%$) of early-type galaxies have old ($\sim$13 Gyr) and coeval GCs. Their variety in the color distribution morphology stems mainly from one parameter, the mean metallicity of a GC system. Furthermore, the color distributions of the remaining ($\sim$30$\%$) GC systems are also explained by the nonlinearity scenario, assuming additional young or intermediate-age GCs with a number fraction of $\sim$20$\%$ of underlying old GCs. Our results reinforce the nonlinearity explanation for the GC color bimodality and provide a new perspective on early-type galaxy formation in the cluster environment, such as the Virgo and Fornax galaxy clusters.