oMEGACat. X. Shedding Light on the Disrupted Dwarf Galaxy of Omega Centauri

Abstract Omega Centauri ( ω Cen) is the most massive and chemically complex star cluster in the Milky Way (MW) and is widely interpreted as the surviving nuclear star cluster of an accreted dwarf galaxy, although the identity of its parent remains uncertain. We test whether the merger remnants Sequoia, Thamnos, and Gaia–Enceladus (GE) originated in a single progenitor, the ω Dwarf, whose nucleus survives as ω Cen. Combining APOGEE and GALAH abundances with Gaia astrometry, we reconstruct the progenitor’s chemical structure using orbital energy as a proxy for premerger radius. The chemically evolved, younger, Al–N-rich population is strongly concentrated toward the center, consistent with formation during or after the merger. By contrast, the primordial population exhibits dwarf-galaxy-like chemistry across all components, supporting a common origin. Its metallicity profile follows an inverted U-shaped radial trend similar to those observed in present-day nucleated dwarf galaxies. The inner regions, traced by ω Cen and Thamnos, are also more α -enhanced than the outskirts, implying shorter and more efficient star formation and supporting a scenario in which the primordial nucleus assembled partly through the inspiral and merger of globular clusters. Although the chemical evidence permits GE to be associated with ω Dwarf, Sequoia and Thamnos alone form a coherent outside-in stripping sequence around ω Cen. Excluding GE yields a mass–metallicity-relation estimate of 6 . 8 − 3.7 + 8.3 × 1 0 8 M ⊙ for ω Dwarf, whereas including GE increases the inferred mass by a factor of ∼3.4, to 2 . 3 − 1.5 + 4.4 × 1 0 9 M ⊙ . Both estimates are consistent with the nuclear-star-cluster–host-galaxy mass relation, identifying Sequoia and Thamnos as the most robust surviving debris of the ω Cen progenitor.