Deriving Metallicities From the Integrated Spectra of Extragalactic Globular Clusters Using the Near-Infrared Calcium Triplet

TL;DR

This study investigates the effectiveness of the near-infrared Calcium Triplet as a metallicity indicator for extragalactic globular clusters, revealing unexpected results and highlighting challenges in its application.

Contribution

The paper provides empirical analysis of CaT as a metallicity indicator for GCs around NGC 1407, highlighting discrepancies and potential limitations of current models.

Findings

Brightest red and blue GCs have similar CaT values despite different colors.

CaT may be influenced by hot blue stars or saturate earlier than models predict.

Color may not linearly trace metallicity in GCs.

Abstract

The Ca triplet (CaT) feature in the near-infrared has been employed as a metallicity indicator for individual stars as well as integrated light of Galactic globular clusters (GCs) and galaxies with varying degrees of success, and sometimes puzzling results. Using the DEIMOS multi-object spectrograph on Keck we obtain a sample of 144 integrated light spectra of GCs around the brightest group galaxy NGC 1407 to test whether the CaT index can be used as a metallicity indicator for extragalactic GCs. Different sets of single stellar population models make different predictions for the behavior of the CaT as a function of metallicity. In this work, the metallicities of the GCs around NGC 1407 are obtained from CaT index values using an empirical conversion. The measured CaT/metallicity distributions show unexpected features, the most remarkable being that the brightest red and blue GCs have similar CaT values despite their large difference in mean color. Suggested explanations for this behavior in the NGC 1407 GC system are: 1) the CaT may be affected by a population of hot blue stars, 2) the CaT may saturate earlier than predicted by the models, and/or 3) color may not trace metallicity linearly. Until these possibilities are understood, the use of the CaT as a metallicity indicator for the integrated spectra of extragalactic GCs will remain problematic.