The Integrated Calcium II Triplet as a Metallicity Indicator: Comparisons with High Resolution [Fe/H] in M31 Globular Clusters

TL;DR

This study evaluates the effectiveness of the Calcium II Triplet as a metallicity indicator in M31 globular clusters, confirming its reliability for clusters older than 2 Gyr with [Fe/H] < -0.4, despite some sensitivity to other elemental abundances.

Contribution

The paper demonstrates that integrated CaT measurements reliably estimate metallicity in distant unresolved clusters, expanding its applicability across a wide metallicity range.

Findings

CaT strength correlates with [Fe/H] within 0.2 dex for certain clusters.

Metal-rich GCs show sensitivity to atomic and molecular line contamination.

CaT can be used for rough metallicity estimates in unresolved clusters.

Abstract

Medium resolution (R=4,000 to 9,000) spectra of the near infrared Ca II lines (at 8498, 8542, and 8662 A) in M31 globular cluster integrated light spectra are presented. In individual stars the Ca II triplet (CaT) traces stellar metallicity; this paper compares integrated CaT strengths to well determined, high precision [Fe/H] values from high resolution integrated light spectra. The target globular clusters cover a wide range in metallicity (from [Fe/H] = -2.1 to -0.2). While most are older than 10 Gyr, some may be of intermediate age (2-6 Gyr). A handful (3-6) have detailed abundances (e.g. low [Ca/Fe]) that indicate they may have been accreted from dwarf galaxies. Using various measurements and definitions of CaT strength, it is confirmed that for GCs with [Fe/H] < -0.4 and older than 2 Gyr the integrated CaT traces cluster [Fe/H] to within about 0.2 dex, independent of age. CaT lines in metal rich GCs are very sensitive to nearby atomic lines (and TiO molecular lines in the most metal rich GCs), largely due to line blanketing in continuum regions. The [Ca/Fe] ratio has a mild effect on the integrated CaT strength in metal poor GCs. The integrated CaT can therefore be safely used to determine rough metallicities for distant, unresolved clusters, provided that attention is paid to the limits of the measurement techniques.