The Age, Metallicity and Alpha-Element Abundance of Galactic Globular Clusters from Single Stellar Population Models

TL;DR

This study evaluates the reliability of single stellar population models in measuring age, metallicity, and alpha-element abundance of Galactic globular clusters, confirming robustness in metallicity and assessing model accuracy for other parameters.

Contribution

It provides a comprehensive comparison of SSP model results with independent measurements, highlighting their strengths and limitations in deriving stellar population parameters.

Findings

Metallicity determinations are highly robust across models.

Ages and alpha-element abundances are accurate for some models.

Application of response functions enables reasonable alpha-element measurements.

Abstract

Establishing the reliability with which stellar population parameters can be measured is vital to extragalactic astronomy. Galactic GCs provide an excellent medium in which to test the consistency of Single Stellar Population (SSP) models as they should be our best analogue to a homogeneous (single) stellar population. Here we present age, metallicity and $\alpha$-element abundance measurements for 48 Galactic globular clusters (GCs) as determined from integrated spectra using Lick indices and SSP models from Thomas, Maraston & Korn, Lee & Worthey and Vazdekis et al. By comparing our new measurements to independent determinations we are able to assess the ability of these SSPs to derive consistent results -- a key requirement before application to heterogeneous stellar populations like galaxies. We find that metallicity determinations are extremely robust, showing good agreement for all models examined here, including a range of enhancement methods. Ages and $\alpha$-element abundances are accurate for a subset of our models, with the caveat that the range of these parameters in Galactic GCs is limited. We are able to show that the application of published Lick index response functions to models with fixed abundance ratios allows us to measure reasonable $\alpha$-element abundances from a variety of models. We also examine the age-metallicity and [$\alpha$/Fe]-metallicity relations predicted by SSP models, and characterise the possible effects of varied model horizontal branch morphology on our overall results.