Stripped Nuclei

Messier 54

Messier 54 (M54) is the nucleus of the Sagittarius dwarf spheroidal galaxy, which is in the process of merging with the Milky Way. Our group has acquired and analysed a large MUSE mosaic of the cluster, which covers about 2.5 times its effective radius, as well as AO assisted WFM and NFM data of its centre. Over 6000 individual stars were extracted from the MUSE datacubes to determine individual stellar metallicities and ages as well as their kinematics (Alfaro-Cuello et al, 2019 & Alfaro-Cuello et al., 2020). These studies revealed that M54 had a complex formation history: young (1-2 Gyr) and metal-rich (0.0 dex) stars with significant rotational support, which are more centrally concentrated, are present on top of a predominant stellar population with old (>10 Gyr), metal-poor (<1.5 dex) and random motion supported stars. M54’s stellar population properties were also studied by analysing its integrated spectrum from the same MUSE data set (Böcker et al., 2020). The extracted age-metallicity distribution from the integrated spectrum alone shows the same complex formation history as the ages and metallicities of the individual stars. On-going work focuses on chemo-dynamical modelling of M54 (Kacharov et al., in preparation) as well as investigations of its potential IMBH (Alfaro-Cuello et al., in preparation). Thus, M54 is an important asset in understanding the formation mechanisms of nuclear star clusters as well as connecting resolved and integrated stellar population analysis.

Omega Centauri

MUSE mosaic on top of the HST/ACS image. The pointings are in yellow (WFM), and blue (WFM-AO) for the more crowded regions, complementing the GTO data (red) out to the half light radius of the cluster (green circle). PI: Nadine Neumayer — MUSE mosaic on top of the *HST*/ACS image. The pointings are in yellow (WFM), and blue (WFM-AO) for the more crowded regions, complementing the GTO data (red) out to the half light radius of the cluster (green circle). PI: Nadine Neumayer

MUSE mosaic on top of the *HST*/ACS image. The pointings are in yellow (WFM), and blue (WFM-AO) for the more crowded regions, complementing the GTO data (red) out to the half light radius of the cluster (green circle). PI: Nadine Neumayer

Omega Centauri (ω Cen) is the most massive globular cluster of the Milky Way and has long been suspected to be a stripped of nuclei of a dwarf galaxy that merged with our Galaxy.
Evidence of that is the complexity of the cluster with multiple stellar populations, multiple sequences in the color-magnitude diagram and large spread in metallicity and age.
In this work we study ω Cen with MUSE and HST data in order to investigate the different stellar populations and its formation history.
We will have information on the metallicity, age, radial velocity and proper motions of thousands of stars allowing us to get valuable information on its formation and merger history with our Galaxy.

Data: MUSE/VLT, HST

For more information, please visit the oMEGACat Homepage.

The assembly of Omega Centauri

The P1, Im and P2 populations of Omega Centauri are identified on the Chromosome Diagram and then mapped onto the age-metallicity relation.See Clontz et al. (2026). — The P1, Im and P2 populations of Omega Centauri are identified on the Chromosome Diagram and then mapped onto the age-metallicity relation. See Clontz et al. (2026).

The P1, Im and P2 populations of Omega Centauri are identified on the Chromosome Diagram and then mapped onto the age-metallicity relation.
See Clontz et al. (2026).

Omega Centauri is the most massive star cluster in the Milky Way and stands out against the Galactic globular cluster population in many ways. Most notably is its large spread in iron. This suggests it is actually the stripped nuclei of a dwarf galaxy which merged with our own Milky Way around 10 billion years ago. Using oMEGACat, a combined astro-photometric (1.4 M; Häberle 2024 ) and spectroscopic metallicity (300k; Nitschai 2023) catalog for the cluster, I have led several projects focused on understanding the formation of the diverse set of stellar populations that constitute this complex cluster. I utilize several probes of Nuclear Star Cluster formation, including:

Age-Metallicity Relation (Clontz et al. 2024)
Chromosome Diagram + Helium Enhancements (Clontz et al. 2025)

My recent work (Clontz et al. 2026) successfully mapped the Chromosome Diagram onto the Age-Metallicity Relation by parsing individual subpopulations from the red giant branch down to the upper main-sequence.

For more information please contact Callie Clontz.

Omega Centauri’s disrupted dwarf galaxy

Metallicity gradient for the proposed progenitor stars in APOGEE DR17 (left) and GALAH DR4 (right). The solid and dashed curves show different functional forms fitted to the combined samples. The magenta solid line is the running median curve.

Metallicity gradient for the proposed progenitor stars in APOGEE DR17 (left) and GALAH DR4 (right). The solid and dashed curves show different functional forms fitted to the combined samples. The magenta solid line is the running median curve.

Omega Centauri is widely considered the surviving nuclear star cluster of an ancient dwarf galaxy accreted by the Milky Way. This project aims to reconstruct the properties and evolutionary history of its disrupted progenitor by combining stellar chemistry, kinematics, orbital dynamics, and age information. Using large spectroscopic and astrometric surveys, including APOGEE, GALAH, and Gaia, we investigate the connection between Omega Centauri and stellar populations distributed throughout the Galactic halo. By tracing chemical enrichment histories, metallicity gradients, and dynamical signatures, we seek to recover the progenitor's original structure, stellar mass, star formation history, and chemical evolution. Ultimately, this approach allows us to investigate how a once-independent galaxy evolved before its disruption, using Omega Centauri as a surviving fossil of its central region.

In Souza et al. 2026, we recovered a structured chemical sequence across the putative progenitor, with chemically primitive populations dominating the outer debris, while the inner regions show evidence for more efficient and prolonged enrichment.

For more information please contact Stefano O. Souza.

The Kinematics of Omega Centauri’s Stellar Populations

Omega Centauri is the likely stripped nucleus of a dwarf galaxy accreted by the Milky Way, and one of the few galactic nuclei we can resolve star by star. Because its relaxation time is longer than its age, the motions of its stars may still remember how the nucleus formed. Using oMEGACat, which combines HST proper motions for 1.4 million stars (Häberle et al. 2024) with MUSE line-of-sight velocities for 300,000 stars (Nitschai et al. 2023), I compare the 3D kinematics of populations defined by their chemistry, metallicity, and age.

Line-of-sight rotation (top) and velocity dispersion (bottom) maps for Omega Centauri’s chromosome-map populations. The primordial P1 stars (right) have a flattened dispersion field, while the enriched P2 and Im stars are round.

Line-of-sight rotation (top) and velocity dispersion (bottom) maps for Omega Centauri’s chromosome-map populations. The primordial P1 stars (right) have a flattened dispersion field, while the enriched P2 and Im stars are round.

Only the chromosome-map populations prove to be dynamically distinct. The primordial (P1) stars have a strongly flattened velocity dispersion field, which we detect independently in both the proper motions and the line-of-sight velocities. The chemically enriched (Im and P2) stars are spherical and more centrally concentrated. Together, these results favor a picture in which the primordial stars kept the angular momentum of the host dwarf galaxy, while the enriched stars formed in compact, centrally concentrated bursts or fell inward as dense clumps (Smith et al., subm.).

For more information please contact Peter Smith.