Nonlinear Color--Metallicity Relations of Globular Clusters. VI. On Calcium II Triplet Based Metallicities of Globular Clusters in Early-type Galaxies

TL;DR

This study evaluates the calcium II triplet (CaT) as a metallicity indicator for globular clusters, revealing its nonlinear behavior at higher metallicities and implications for interpreting GC metallicity distributions in galaxies.

Contribution

The paper provides new stellar population synthesis models demonstrating the nonlinear relation between CaT strength and metallicity, especially in metal-rich regimes.

Findings

CaT strength correlates linearly with [Fe/H] below -0.5 for old GCs

CaT does not increase with [Fe/H] in the metal-rich regime

Nonlinear CaT-metallicity relation explains observed bimodal color and index distributions

Abstract

The metallicity distribution function of globular clusters (GCs) in galaxies is a key to understanding galactic formation and evolution. The calcium II triplet (CaT) index has recently become a popular metal abundance indicator thanks to its sensitivity to GC metallicity. Here we revisit and assess the reliability of CaT as a metallicity indicator using our new stellar population synthesis simulations based on empirical, high-resolution fluxes. The model shows that the CaT strength of old ($>$ 10 Gyr) GCs is proportional to ${\rm [Fe/H]}$ below $-0.5$. In the modest metal-rich regime, however, CaT does not increase anymore with ${\rm [Fe/H]}$ due to the little contribution from coolest red giant stars to the CaT absorption. The nonlinear nature of the color--$CaT$ relation is confirmed by the observations of GCs in nearby early-type galaxies. This indicates that the CaT should be used carefully when deriving metallicities of metal-rich stellar populations. Our results offer an explanation for the observed sharp difference between the color and $CaT$ distributions of GCs in the same galaxies. We take this as an analogy to the view that metallicity--color and metallicity--Lick index nonlinearity of GCs is primarily responsible for their observed "bimodal" distributions of colors and absorption indices.