Full spectral fitting of Milky Way and M31 globular clusters: ages and metallicities

TL;DR

This study uses spectral fitting to determine ages and metallicities of globular clusters in the Milky Way and M31, revealing distinct formation histories and metallicity distributions.

Contribution

It introduces a modern pixel-to-pixel spectral fitting method applied to a large sample of clusters, providing new insights into their age-metallicity relations and distribution differences.

Findings

M31 globular clusters range from 150 Myr to the universe's age.

Galactic GCs show metallicity bimodality, M31's MDF is not clearly bimodal.

Distinct star formation and accretion histories inferred for both galaxies.

Abstract

Context: The formation and evolution of disk galaxies are long standing questions in Astronomy. Understanding the properties of globular cluster systems can lead to important insights on the evolution of its host galaxy. Aims: We aim to obtain the stellar population parameters - age and metallicity - of a sample of M31 and Galactic globular clusters. Studying their globular cluster systems is an important step towards understanding their formation and evolution in a complete way. Methods: Our analysis employs a modern pixel-to-pixel spectral fitting technique to fit observed integrated spectra to updated stellar population models. By comparing observations to models we obtain the ages and metallicities of their stellar populations. We apply this technique to a sample of 38 globular clusters in M31 and to 41 Galactic globular clusters, used as a control sample. Results: Our sample of M31 globular clusters spans ages from 150 Myr to the age of the Universe. Metallicities [Fe/H] range from -2.2 dex to the solar value. The age-metallicity relation obtained can be described as having two components: an old population with a flat age-[Fe/H] relation, possibly associated with the halo and/or bulge, and a second one with a roughly linear relation between age and metallicity, higher metallicities corresponding to younger ages, possibly associated with the M31 disk. While we recover the very well known Galactic GC metallicity bimodality, our own analysis of M31's metallicity distribution function (MDF) suggests that both GC systems cover basically the same [Fe/H] range yet M31's MDF is not clearly bimodal. These results suggest that both galaxies experienced different star formation and accretion histories.