Stefano O. Souza (Max Planck Institute for Astronomy, Germany)
Omega Centauri is the most massive and chemically complex star cluster in the Milky Way and has long been regarded as the surviving nucleus of an accreted dwarf galaxy. However, the identity and structure of its parent dwarf galaxy remain debated. In this talk, I will revisit the origin of Omega Centauri by placing its multiple stellar populations in the broader context of the surrounding stellar halo debris. Combining APOGEE and GALAH abundances with Gaia-based orbital information, we compare Omega Centauri with the retrograde substructures Thamnos, Sequoia, and Gaia-Enceladus. We find that the system comprising Omega Centauri and these halo substructures can be interpreted as the remnant of a massive, disrupted dwarf galaxy, which we refer to as the Omega Dwarf. By analysing its chemical gradients, we show that the inner regions of this dwarf galaxy host a chemically evolved, Al-N-He-rich population whose concentration increases toward the centre, while the outskirts are dominated by a more primordial, dwarf-galaxy-like population. We also find an inverted U-shaped metallicity profile, enhanced alpha-abundances in the inner regions, and neutron-capture patterns that point to delayed and spatially dependent enrichment. These results suggest that the nucleus, which survives today as Omega Centauri, likely formed through a combination of in situ delayed star formation and merger of inspiraling old globular clusters. More broadly, our findings indicate that the Milky Way’s last major merger may have involved a more structurally and chemically complex dwarf galaxy than previously assumed, and some halo substructures are actually different merger stages of a unique dwarf galaxy.
Rameshan Thimmappa
