oMEGACat Population Explorer

ωCen's five chemically distinct stellar populations are the fossil record of a consumed dwarf galaxy. Select a population to explore its metallicity, age, abundance ratios, and what it implies about the assembly history of NGC 5139.

🔬 Peer-Reviewed Survey ⚠ Boundaries Approximate
Why does ωCen have multiple stellar populations? Normal globular clusters show a narrow metallicity spread (~0.1 dex). ωCen spans −2.2 to −0.5 in [Fe/H] — a 1.7 dex range that implies repeated star-formation episodes fuelled by retained supernova ejecta. Only a dwarf galaxy nucleus, deep enough in a potential well to hold its own gas, could do this. The oMEGACat survey (2023–2026) used machine-learning on MUSE IFU spectra to tag ~300,000 stars across five populations.
Stellar Populations — click to explore
Metallicity Distribution Function

Approximate [Fe/H] distribution per population. Based on Johnson & Pilachowski 2010 + oMEGACat 2026 ML tagging.

Select a population above

Click any population in the list to see its chemical properties and assembly interpretation.

Fraction of Total Cluster Mass by Population

Approximate mass fractions from Bellini et al. 2017 proper-motion cleaned samples + oMEGACat photometric membership.

oMEGACat is a multi-epoch HST photometric and MUSE spectroscopic survey of ωCen covering ~300,000 stars. The IX instalment (Clontz et al. 2026) used a random-forest classifier trained on known [Fe/H], [Mg/Fe], and [Ca/Fe] values to assign probabilistic population membership to stars with only photometric data — extending chemical tagging to stars too faint for individual spectroscopy.

Population naming follows the Johnson & Pilachowski (2010) scheme: Metal-Poor (MP), Intermediate-1, Intermediate-2, Metal-Rich (MR), and the anomalous "supermetal-rich" population (RGB-a). Each formed in a distinct star-formation episode, with the later populations enriched by ejecta from earlier ones.

The dwarf galaxy connection: the multi-episode chemical history, the presence of an IMBH, the retrograde orbit in the Galaxy (Dinescu et al. 1999), the flattened morphology, and the stellar population age spread (1–3 Gyr between oldest and youngest populations) all point to ωCen as the stripped nucleus of a dwarf galaxy accreted by the Milky Way.

References: Clontz et al. 2026 (arXiv:2603.01041) · Nitschai et al. 2023 ApJS 268:40 · Johnson & Pilachowski 2010 ApJ 722:1373 · Bellini et al. 2017 ApJ 842:6