Cluster Ages to Reconstruct the Milky Way Assembly (CARMA). II. The age-metallicity relation of Gaia-Sausage-Enceladus globular clusters

TL;DR

This study determines the ages of 13 globular clusters linked to the Gaia-Sausage-Enceladus merger, revealing a well-defined age-metallicity relation with two distinct formation epochs, shedding light on the Milky Way's assembly history.

Contribution

It provides homogeneous age estimates for GSE-associated clusters using HST data, identifying two major star formation episodes and potential origins of specific clusters, advancing understanding of galaxy merger events.

Findings

Majority of clusters follow a clear age-metallicity relation.

Two distinct globular cluster formation epochs separated by ~2 Gyr.

Clusters NGC 288 and NGC 6205 are older, likely in-situ origins.

Abstract

We present the age determination of 13 globular clusters dynamically associated with the Gaia-Sausage-Enceladus (GSE) merger event, as part of the CARMA project effort to trace the Milky Way assembly history. We used deep and homogeneous archival $Hubble$ $Space$ $Telescope$ data, and applied isochrone-fitting to derive homogeneous age estimates. We find that the majority of the selected clusters form a well-defined age-metallicity relation, with a few outliers. Among these, NGC 288 and NGC 6205 are more than 2 Gyr older than the other GSE globular clusters at similar metallicity, and are therefore interpreted as of likely in-situ origin. Moreover, NGC 7099 is somewhat younger than the average GSE trend, this suggesting a possible alternative dwarf galaxy progenitor, while NGC 5286 is mildly older, as if its progenitor was characterised by an higher star-formation efficiency. Another remarkable feature of the resulting age-metallicity relation is the presence of two epochs of globular cluster formation, with a duration of $\sim0.3$ Gyr each and separated by $\sim2$ Gyr. These findings are in excellent agreement with the age-metallicity relation of halo field stars found by Gonz\'alez-Koda et al., clearly hinting at episodic star-formation in GSE. The age of the two formation epochs is similar to the mean age of the two groups of in-situ globular clusters previously studied by CARMA. These epochs might therefore be precisely pinpointing two important dynamical events that GSE had with the Milky Way during its evolutionary history. Finally, we discuss the correlation between the recent determination of Si and Eu with the clusters age and origin.