Date: 16.06.2026

Stefano Oliveira de Souza (MPI,DE)

Reconstructing the Milky Way's last major merger that left behind Omega Centauri

Abstract:
The stellar halo of the Milky Way preserves the fossil record of past accretion events, but disentangling the debris of ancient mergers remains challenging once their dynamical signatures have phase-mixed. In this talk, I will present a chemodynamical reconstruction of a possible disrupted progenitor associated with Omega Centauri, which we refer to as the Omega Dwarf. Using chemical abundances from APOGEE and GALAH, together with Gaia astrometry, we investigate whether Sequoia, Thamnos, Gaia-Enceladus, and Omega Centauri can be interpreted as different regions of a single accreted dwarf galaxy. We recover a structured chemical sequence across the putative progenitor, with chemically primitive populations dominating the outer debris, while the inner regions show evidence of more efficient and prolonged enrichment. Sequoia naturally fits an outside-in formation, with Omega Centauri as the nucleus. At the same time, Thamnos is likely a more recent stripping event, resembling chemical features present only in Omega Centauri. On the other hand, the Gaia-Enceladus association remains uncertain. This framework provides a new way to connect stellar halo substructures with the internal chemical anatomy of their disrupted parent galaxies.