Cluster Ages to Reconstruct the Milky Way Assembly (CARMA)

III. NGC 288 as the first Splashed globular cluster

¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
² Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
³ atlanTTic, Universidade de Vigo, Escola de Enxeñaría de Telecomunicación, 36310 Vigo, Spain
⁴ Universidad de La Laguna, Avda. Astrofísico Fco. Sánchez, 38205 La Laguna, Tenerife, Spain
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 INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone, Italy
⁸ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁹ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 3 March 2025
Accepted: 21 July 2025

Abstract

The system of globular clusters (GCs) in the Milky Way (MW) comprises a mixture of both in situ and accreted clusters. Tracing the origin of GCs provides invaluable insights into the formation history of the MW. However, reconciling diverse strands of evidence is often challenging. A notable example is NGC 288, where despite significant efforts in the literature, the available chrono-chemodynamical data have yet to provide a definitive conclusion regarding its origin. On the one hand, all post-Gaia dynamical studies indicate an accreted origin for NGC 288, pointing towards its formation taking place in the Gaia-Sausage-Enceladus (GSE) dwarf galaxy. On the other hand, NGC 288 has been found to be 2.5 Gyr older than other GSE GCs at the same metallicity, suggesting a different (and possibly in situ) origin. In this work, we address the unresolved question on the origin of NGC 288 by analysing its chrono-chemical properties in an unprecedentedly homogeneous framework. First, we compared the location of NGC 288 in the age-metallicity plane with that of other two GCs at similar metallicity, namely, NGC 6218 and NGC 6362, whose chemodynamical properties unambiguously identify them as in situ. The age estimates obtained within the homogeneous framework of the CARMA collaboration show that the three clusters are coeval, reinforcing the contrast with the dynamical interpretation. Then, we derived the chemical composition of NGC 288 using UVES-FLAMES at VLT high-resolution spectroscopic archival data and compared the abundances with a sample of in situ and accreted clusters at similar metallicity. We found a consistency with the chemistry of in situ systems, especially in Si, Ti, Zn, and abundance ratios relative to Eu. To reconcile these results with its orbital properties, we propose a scenario where NGC 288 formed in the proto-disc of the MW and was then dynamically heated by the interaction with the GSE merger. This is a fate that resembles that of proto-disc stars undergoing the so-called Splash event. Therefore, NGC 288 demonstrates the importance of a homogeneous chrono-chemodynamical information in the interpretation of the origin of MW GCs.