On the metallicity-color relations and bimodal color distributions in extragalactic globular cluster systems

We perform a series of numerical experiments to study how the nonlinear metallicity-color relations predicted by different stellar population models affect the color distributions observed in extragalactic globular cluster systems. We present simulations in the UBVRIJHK bandpasses based on five different sets of simple stellar population ( SSP)models. The presence of photometric scatter in the colors is included as well. We find that unimodal metallicity distributions frequently project'' into bimodal color distributions. The likelihood of this effect depends on both the mean and dispersion of the metallicity distribution, as well as of course on the SSP model used for the transformation. Adopting the Teramo-SPoT SSP models for reference, we find that optical-near-IR colors should be favored with respect to other colors to avoid the bias effect in globular cluster color distributions discussed by Yoon and coworkers. In particular, colors such as ( V - H) or ( V - K) are more robust against nonlinearity of the metallicity-color relation, and an observed bimodal distribution in such colors is more likely to indicate a true underlying bimodal metallicity distribution. Similar conclusions come from the simulations based on different SSP models, although we also identify exceptions to this result.