Understanding the formation and evolution of the centers of galaxies is one of the most intriguing challenges in astrophysics. The new study aims to unveil the physical processes that led to the formation of two striking features surrounding the black holes at the centers of most galaxies: nuclear star clusters and nuclear stellar disks. Only in the last decade have such galactic structures been observed in the Milky Way and extragalactic systems.
Observationally, these two structures were viewed as products of separate formation processes, with observational surveys showing no clear correlation between their masses and sizes. However, the formation process is not understood, and realistic simulations have so far been lacking.
This long-standing mystery is now challenged by a new galaxy simulation from the SMUGGLE-Ring project, offering a fresh perspective and bridging theory and observations. In a paper accepted as a Letter to the Editor in Astronomy & Astrophysics, AIP researcher Dr. SungWon Kwak and collaborators demonstrate, for the first time, that a fully self-consistent, high-resolution hydrodynamical simulation of a Milky Way–like barred galaxy can naturally form both a nuclear star cluster and a nuclear stellar disk and follow their growth over billions of years. The paper is available on the arXiv preprint server.
The bar feeds both structures
The simulation reveals that the galaxy’s stellar bar plays a central role in this process. “Our simulation achieves this by showing how the galactic bar acts like a cosmic conveyor belt, channeling gas inward to feed both structures simultaneously from the same reservoir,” Kwak explains. As gas accumulates in the central region, stellar feedback from dying stars generates shocks that repeatedly trigger new episodes of star formation. Over the course of several billion years, hundreds of millions of solar masses of stars are assembled in these central structures.
One of the key advantages of the simulation is that it allows researchers to observe processes that cannot be directly seen in real galaxies. Astronomical observations provide only a single snapshot of a galaxy at the present day. By contrast, the simulation follows the evolution of the galaxy over four billion years, allowing scientists to watch the stellar bar form, trace the inward flow of gas, monitor bursts of star formation and observe how the nuclear stellar disk grows outward from the center over time.
Why the link looks hidden
The results also explain why observations have struggled to reveal a clear connection between nuclear star clusters and nuclear stellar disks. “The apparent disconnection does not mean that the stars themselves differ fundamentally in age, chemical composition, or motion,” explains Dr. Cristina Chiappini, also an AIP scientist and co-author of the study.
Instead, the simulation shows that the structural relationship between the two components naturally evolves over time. During long periods of steady growth, the relative masses and sizes of the cluster and the disk gradually drift apart. As a result, the nuclear star clusters and nuclear stellar disks of galaxies observed at different stages of their evolution can appear remarkably different, even if the underlying growth mechanism is the same.
Live dark matter changes the picture
Including realistic dark matter dynamics in the simulation plays a crucial role in this finding. “Previous studies rely on fixed background potentials for the galactic bar and dark matter halo, but the realistic dynamical treatment of stars and the dark matter halo using live particles in our model allows us to form a realistic bar that evolves over time and then naturally forms nuclear structures,” explains Dr. Ivan Minchev, co-author of the study.
“Furthermore, our model also exhibits a ‘dark gap’ around the bar region, which is found in many observations and is known as evidence of the interaction between stars and dark matter driven by the rotation of the stellar bar.”
A merger may reshape the core
The picture becomes even more fascinating because, in the simulation, a particularly massive star cluster with roughly 30 million solar masses spirals into the galactic center and merges with the nuclear star cluster. Interestingly, recent observations have captured such massive star clusters inside the bar of NGC 1365, some of which are expected to spiral into its center and merge with the galaxy’s nuclear star cluster. Such merger events can alter the mass and size of the nuclear star cluster over a short time scale.
This makes the coevolution history of galactic centers more complex, yet interesting, since a supermassive black hole is lurking inside the nuclear star cluster in galaxies. Consequently, these merger events might leave an imprint on the mass of the supermassive black hole, potentially expanding the connection between galactic components and allowing us to interpret future observations.
Publication details
SungWon Kwak et al, SMUGGLE-Ring: Evolutionary link between nuclear star cluster and nuclear disk, arXiv (2026). DOI: 10.48550/arxiv.2606.05157
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Leibniz Institute for Astrophysics Potsdam
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Milky Way-like simulation reveals how central galactic structures grow together (2026, August 6)
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