Optimal Integrated Abundances for Chemical Tagging of Extragalactic Globular Clusters

TL;DR

This study evaluates the systematic uncertainties in integrated light spectroscopy of globular clusters to identify stable abundance ratios, aiding chemical tagging of unresolved extragalactic clusters.

Contribution

It quantifies uncertainties in elemental abundances from integrated light spectra and identifies the most stable ratios for future extragalactic cluster analysis.

Findings

Uncertainties are ~0.1-0.2 dex with well-modeled populations.

[Ca I/Fe I] is the most stable alpha-element ratio (<0.1 dex).

[Ba II/Eu II] ratios are stable and useful for chemical tagging.

Abstract

High resolution integrated light (IL) spectroscopy provides detailed abundances of distant globular clusters whose stars cannot be resolved. Abundance comparisons with other systems (e.g. for chemical tagging) require understanding the systematic offsets that can occur between clusters, such as those due to uncertainties in the underlying stellar population. This paper analyses high resolution IL spectra of the Galactic globular clusters 47 Tuc, M3, M13, NGC 7006, and M15 to (1) quantify potential systematic uncertainties in Fe, Ca, Ti, Ni, Ba, and Eu and (2) identify the most stable abundance ratios that will be useful in future analyses of unresolved targets. When stellar populations are well-modelled, uncertainties are ~0.1-0.2 dex based on sensitivities to the atmospheric parameters alone; in the worst case scenarios, uncertainties can rise to 0.2-0.4 dex. The [Ca I/Fe I] ratio is identified as the optimal integrated [alpha/Fe] indicator (with offsets <0.1 dex), while [Ni I/Fe I] is also extremely stable to within <0.1 dex. The [Ba II/Eu II] ratios are also stable when the underlying populations are well modelled and may also be useful for chemical tagging.