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

We present the age determination of 13 globular clusters that are dynamically associated with the Gaia-Sausage-Enceladus (GSE) merger event, as part of the CARMA project's 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 a similar metallicity, and are therefore interpreted as probably having originated in situ. Moreover, NGC 7099 is somewhat younger than the average GSE trend, which suggests a possible alternative dwarf galaxy progenitor, while NGC 5286 is slightly older, as if its progenitor was characterised by greater 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 ∼0.3 Gyr each and separated by ∼2 Gyr. These findings are in excellent agreement with the age-metallicity relation recently found for halo field stars, which clearly hints 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 precisely pinpoint two important dynamical events that Gaia-Sausage-Enceladus had with the Milky Way during its evolutionary history. Finally, we discuss the correlation between the recent spectroscopic determination of Si and Eu, and the clusters age and origin.