Revealing the Origins of Galactic Globular Clusters via their Mg–Al Abundances

Abstract Many Galactic globular clusters (GCs) originated in diverse host galaxies before being subsequently incorporated into the Milky Way through hierarchical galaxy assembly. Identifying their origins is crucial for revealing galaxy properties at early times. Traditional classification methods relying on dynamical properties face inherent uncertainties stemming from the evolving Galactic potential and complex merger histories. Chemically driven classification confronts a distinct obstacle: multiple populations—abundance variations in light elements of GC members. In this Letter, we identify primordial populations exhibiting lower [Al/Fe] as reliable tracers of their birth environments’ chemical evolution. A clear chemical dichotomy emerges between in situ and accreted GC populations at [Fe/H] > −1.5, particularly in the [Mg/Fe]–[Al/Fe] plane, indicating that their progenitor galaxies have experienced fundamentally different enrichment histories. While our chemically driven classification demonstrates general consistency with dynamically driven classifications, notable discrepancies emerge: NGC 288 and M4 are reclassified as in situ, and Terzan 9 as accreted. This chemically driven GC classification provides a promising application for Galactic archaeology.