On the Metallicity-Color Relations and Bimodal Color Distributions in Extragalactic Globular Cluster Systems

TL;DR

This study investigates how nonlinear metallicity--color relations can cause unimodal metallicity distributions to appear bimodal in color, emphasizing the importance of choosing appropriate colors like V-H or V-K for accurate interpretation.

Contribution

The paper demonstrates through simulations that certain optical--near-IR colors are more reliable for detecting true metallicity bimodality in globular clusters, considering different stellar population models.

Findings

Unimodal metallicity distributions can project into bimodal color distributions due to nonlinear relations.

Colors such as V-H and V-K are less affected by nonlinearity, making bimodal color distributions more indicative of true metallicity bimodality.

Results are consistent across different SSP models, with some exceptions.

Abstract

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--to--near-IR colors should be favored with respect to other colors to avoid the bias effect in globular cluster color distributions discussed by \citet{yoon06}. In particular, colors such as \vh\ or \vk 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.