The SLUGGS Survey: Measuring globular cluster ages using both photometry and spectroscopy

TL;DR

This paper introduces a new method combining photometry and spectroscopy to accurately determine globular cluster ages, revealing diverse formation histories in different galaxies and challenging the assumption of a universal colour-metallicity relation.

Contribution

The paper presents a novel technique for measuring globular cluster ages using combined photometry and spectroscopy, applied to multiple galaxies for the first time.

Findings

Galaxies show diverse globular cluster age distributions.

Globular cluster colour-metallicity relation is not universal.

Correlation between metal-rich globular cluster ages and field star ages.

Abstract

Globular cluster ages provide both an important test of models of globular cluster formation and a powerful method to constrain the assembly history of galaxies. Unfortunately, measuring the ages of unresolved old stellar populations has proven challenging. Here, we present a novel technique that combines optical photometry with metallicity constraints from near-infrared spectroscopy in order to measure ages. After testing the method on globular clusters in the Milky Way and its satellite galaxies, we apply our technique to three massive early-type galaxies using data from the SLUGGS Survey. The three SLUGGS galaxies and the Milky Way show dramatically different globular cluster age and metallicity distributions, with NGC 1407 and the Milky Way showing mostly old globular clusters while NGC 3115 and NGC 3377 show a range of globular ages. This diversity implies different galaxy formation histories and that the globular cluster optical colour-metallicity relation is not universal as is commonly assumed in globular cluster studies. We find a correlation between the median age of the metal rich globular cluster populations and the age of the field star populations, in line with models where globular cluster formation is a natural outcome of high intensity star formation.