Cluster Ages to Reconstruct the Milky Way Assembly (CARMA)

II. The age-metallicity relation of Gaia–Sausage–Enceladus globular clusters

¹ AtlanTTic, Universidade de Vigo, Escola de Enxeñaría de Telecomunicación, 36310 Vigo, Spain
² Universidad de La Laguna, Avda. Astrofísico Fco. Sánchez, 38205 La Laguna, Tenerife, Spain
³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁴ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Roma, Italy
⁵ INAF – Osservatorio Astronomico di Abruzzo, Via M. Maggini, 64100 Teramo, Italy
⁶ INFN – Sezione di Pisa, Università di Pisa, Largo Pontecorvo 3, 56127 Pisa, Italy
⁷ Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, 38206 La Laguna, Tenerife, Spain
⁸ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Piero Gobetti 93/2, 40129 Bologna, Italy
⁹ Universidad de Granada, Departamento de Física Teórica y del Cosmos, Campus Fuente Nueva, Edificio Mecenas, 18071 Granada, Spain
¹⁰ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹¹ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK

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Received: 25 February 2025
Accepted: 17 June 2025

Abstract

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.