Exploring the Kinematics of Omega Centauri’s Metallicity Populations
Omega Centauri (Omega Cen) is the largest known globular cluster in the Milky Way, but it is far from a simple collection of stars. Unlike typical globular clusters, which contain stars of similar ages and chemical compositions, Omega Cen has multiple populations of stars with different metallicities—meaning the amount of elements heavier than hydrogen and helium varies among them. This unusual characteristic has led astronomers to believe that Omega Cen may be the remnant of a dwarf galaxy that was captured and absorbed by the Milky Way. One mystery within Omega Cen is its metal-rich population, which does not show the same chemical patterns as its metal-poor counterparts. Some astronomers suspect that this metal-rich group of stars may have been accreted—meaning they were once part of another system that merged with Omega Cen. To explore this idea, Vernekar et al. (2025) analyze the motions (kinematics) of different stellar populations within Omega Cen using high-precision data from the Gaia satellite and the Hubble Space Telescope (HST).
Methods
The researchers use newly released data from Gaia's Focused Product Release (FPR) and Data Release 3 (DR3), which provide precise measurements of stellar positions and movements. To study the cluster’s chemical composition, they incorporate metallicity data from the oMEGACat I catalog, which contains spectroscopic measurements of over 300,000 stars in Omega Cen. The team categorizes stars into four metallicity groups based on their iron abundance ([Fe/H]) and examines their motions to determine whether the metal-rich stars move differently from the rest of the cluster. By analyzing the stars’ proper motions—how they shift in the sky over time—the team searches for patterns that might indicate whether the metal-rich stars were once separate from Omega Cen or have been fully incorporated into its structure. They also use Hubble's oMEGACat II catalog, which provides even more precise motion data in the cluster’s core, to verify their findings. Finally, they assess the orbital anisotropy of the populations, looking at whether their motions are more radial (moving in and out) or tangential (moving in circles).
Results
Despite previous speculation that the metal-rich stars were once separate from Omega Cen, the researchers find no significant differences in the motions of the four populations. The metal-rich stars move in a similar way to the other populations, suggesting they are well-mixed into the cluster. This challenges the idea that they were accreted separately and instead supports the possibility that they formed within Omega Cen itself. Interestingly, the study confirms that all populations within Omega Cen show signs of rotation, meaning the cluster as a whole is spinning. This rotation is consistent across the different metallicity groups, further supporting the idea that these stars have been part of the cluster for a long time rather than recently added.
Additional Analysis
To strengthen their findings, the team cross-checks their results with Hubble data from the oMEGACat II catalog, which provides even more precise stellar motion measurements in the core of the cluster. Even with this higher precision, no significant differences in movement between the populations are found. Finally, the researchers investigate the orbital anisotropy of the stars, which describes whether their motions are more radial or tangential. They find that all populations exhibit similar behavior, further reinforcing the conclusion that Omega Cen is a well-mixed system.
Conclusions
This study provides strong evidence that the metal-rich stars in Omega Cen are not dynamically distinct from the rest of the cluster. Instead of being a recently merged population, they seem to have been present in Omega Cen for a long time, possibly forming through self-enrichment—where earlier generations of stars enriched later ones with heavier elements. While the origins of Omega Cen remain an open question, this research helps rule out one possible scenario. Future data releases from Gaia and the Vera C. Rubin Observatory's LSST could provide even more detailed motion measurements, helping astronomers further unravel the history of this fascinating cluster.
Source: Vernekar
