The formation of globular cluster systems in massive elliptical galaxies: Globular cluster multimodality from radial variation of stellar populations

The most massive elliptical galaxies show a prominent multimodality in their globular cluster system color distributions. Understanding the mechanisms that lead to multiple globular cluster subpopulations is essential for a complete picture of massive-galaxy formation. By assuming that globular cluster formation traces the total star formation and taking into account the radial variations in the composite stellar populations predicted by the Pipino and Matteucci multizone photochemical evolution code, we compute the distribution of globular cluster properties as a function of galactocentric radius. We compare our results to the spectroscopic measurements of globular clusters in nearby early-type galaxies by Puzia and coworkers and show that the observed multimodality in globular cluster systems of massive ellipticals can be ascribed to the radial variation in the mix of stellar populations. Our model predicts the presence of a super-metal-rich population of globular clusters in the most massive elliptical galaxies, which is in very good agreement with the spectroscopic observations. The size of this high-metallicity population scales with galaxy mass, in the sense that more massive galaxies host larger such cluster populations. We predict an increase of mean metallicity of the globular cluster systems with host galaxy mass, and forecast that older clusters exhibit lower metallicities and higher alpha/Fe ratios. We find that a nonlinear color-metallicity relation may be partly responsible for a color multimodality. On the other hand, the formation of globular clusters from subsequently accreted gas, either with primordial abundances or solar metallicity, delivers model predictions that are at variance with the observations.