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 and is widely regarded as the surviving nuclear star cluster of an accreted dwarf galaxy. However, its parent host remains uncertain. Here, we investigate a scenario in which Sequoia, Thamnos, and Gaia--Enceladus (GE) are debris from a single disrupted progenitor, the $ω\,$Dwarf, whose nucleus survives today as $ω\,$Cen. Using APOGEE and GALAH abundances together with Gaia astrometry, we reconstruct the chemical structure across this progenitor adopting orbital energy as a proxy for pre-merger radius. We find that the chemically evolved (younger Al-N-He-rich) population is strongly concentrated toward the inner regions, representing a population formed after/during the merger, while the primordial population represents a dwarf-galaxy-like population, supporting a common dwarf-galaxy origin for its components. The metallicity profile shows an inverted U-shaped gradient similar to those observed in present-day nucleated dwarf galaxies. At the same time, the inner regions ($ω\,$Cen+Thamnos) are more $α$-enhanced than the outskirts, pointing to shorter and more efficient star formation and indicating that the nucleus may have assembled through the merger of inspiraling globular clusters. Neutron-capture abundances reveal a Eu-rich, r-process-dominated outskirts and inner regions enhanced in [Ba/Eu] and [La/Eu], requiring delayed enrichment and more complex chemical evolution. Finally, our analysis shows that Sequoia and Thamnos naturally fit an outside-in stripping sequence around $ω\,$Cen, whereas the connection with GE remains unsure.

Figures (17)

• Figure 1: Schematic illustration of the proposed $\omega$ Dwarf. On the left, the structure of the progenitor $\omega$ Dwarf galaxy is displayed, with the disc structure composed of an outer stellar component associated with Sequoia (green), an intermediate component corresponding to Gaia--Enceladus (yellow), an inner component identified with Thamnos (red), and a dense central nuclear star cluster (blue). In the middle, an illustration of the disruption of this system after its infall into the proto-Milky Way ($\rm t_i$), producing debris associated with each halo substructure mentioned before, as well as the tidal debris of $\omega$ Cen observed today ($\rm t_f$). The infall process is also explained in the right as a flowchart. Note: for illustration purposes, not to scale. This figure is inspired by figures in Koppelman2020 and Skuladottir2025.
• Figure 2: $\omega$ Dwarf stars with $P_{\omega\,\mathrm{Dwarf}} > 70\%$. The top row shows the APOGEE sample, and the bottom row shows the GALAH sample. The left panels are the orbital energy ($E$) as a function of the vertical component of angular momentum ($L_z$). Black points trace the MW population. Colored symbols identify stars in Sequoia (green squares), Gaia--Enceladus (orange circles), Thamnos (red triangles), and $\omega$ Cen (blue stars). The middle panels present the orbital-action space in a diamond projection. The right panels show the chemical plane [$\mathrm{Mg}/\mathrm{Mn}$] versus [$\mathrm{Al}/\mathrm{Fe}$], with unevolved stars in the left subpanels delimited by the dashed line and evolved stars in the right subpanels.
• Figure 3: Chemical pattern of $\omega$ Dwarf for the elements provided by both APOGEE and GALAH. Upper panel: the violins represent the distribution obtained from the data. For each element, the left panel shows the distribution from APOGEE, while the right panel shows that from GALAH. The distribution for each $\omega$ Dwarf substructure is represented using the same colour-code as in \ref{['fig:target_selection']}. The dashed grey line is the solar chemical pattern. Bottom panel: the star-by-star median difference for the intersection sample APOGEE--GALAH. The black squares represent the medians considering all intersection stars, while the colored symbols and lines show the medians for the individual populations.
• Figure 4: Orbital distribution in action space for stars associated with $\omega$ Dwarf and Nephele Pagnini2025Pagnini2025b. Black points show $\omega$ Dwarf members with membership probability $>70\%$. Blue points represent the Nephele stars assuming the same probability floor, $P_{\rm Nephele}>70\%$. The red dots are the Nephele stars in common with $\omega$ Dwarf.
• Figure 5: Stellar populations across $\omega$ Dwarf in the [Al/Fe]--[Fe/H] plane for APOGEE. The upper panel represents the entire sample of $\omega$ Dwarf members, whose symbols and colours follow the same definition as in \ref{['fig:target_selection']}. Each $\omega$ Dwarf substructure is individually displayed in the bottom panels. The lines show the limits for the lower (P1) and upper (P2) streams as defined by Dondoglio2025, while the dotted lines show the limits for the unknown population.