A high-resolution computer simulation has revealed how two distinct structures at the centres of galaxies may grow from the same supply of gas. The model shows a compact nuclear star cluster and a surrounding nuclear stellar disk developing together as a galactic bar continuously channels material towards the centre.
The research, led by SungWon Kwak and accepted for publication as a Letter in Astronomy & Astrophysics, follows the evolution of an isolated Milky Way-mass galaxy for 4 billion years. The findings suggest that the apparent differences between nuclear star clusters and nuclear stellar disks do not necessarily mean that they formed independently.
A stellar bar feeds the galactic centre
The researchers constructed a galaxy containing a stellar disk, a gas disk, a central bulge and a live dark matter halo. Unlike models that impose a fixed bar-shaped gravitational field, this simulation allowed the bar to form and evolve naturally through interactions among stars, gas and dark matter.
The model used the SMUGGLE framework, which represents the interstellar medium as a mixture of hot, warm and cold gas. It also includes several forms of stellar feedback, including supernova explosions, radiation from young massive stars and stellar winds.
A stellar bar emerged approximately 1 billion years after the simulation began. It subsequently became stronger and extended to a length of about 5 kiloparsecs, comparable to estimates for the Milky Way's bar. The bar's gravitational influence removed angular momentum from gas and directed it towards the inner kiloparsec of the galaxy.
This sustained inflow created a reservoir from which both central stellar structures could grow. The simulation therefore presents the galactic bar as a mechanism connecting the larger disk of the galaxy with its much smaller nuclear region.
Two structures grow from the same gas
A nuclear stellar disk is a flattened, rotationally supported concentration of stars near a galaxy's centre. A nuclear star cluster, by comparison, is a denser and more compact structure in which the stars have less ordered motion.
To separate these structures from older stars already present in the simulated galaxy, the researchers analysed stars born after the bar formed. They identified the nuclear star cluster dynamically as the pressure-supported central region, which generally remained within approximately 100 parsecs of the centre.
The surrounding nuclear stellar disk followed an inside-out growth pattern. Young stars formed mainly near its outer edge, causing the disk to expand as the simulation progressed. Its radius increased from about 210 parsecs to approximately 510 parsecs, while the nuclear star cluster remained comparatively compact.
The masses and star-formation rates of the two structures generally evolved in parallel. This indicates that both were being supplied by the same bar-driven gas flow, even though their sizes, shapes and stellar motions remained different.
Stellar feedback produces repeated shocks
The inward movement of gas was not smooth. Energy and momentum released by young and dying stars repeatedly disturbed the nuclear gas disk, producing cavities and shocks. These events redistributed angular momentum and pushed gas from the disk's outer region towards the central star cluster.
Each major disturbance was followed by a rapid increase in star formation. Similar shock-driven inflows have been identified observationally in barred galaxies, giving the simulated process a connection to structures seen in real galactic centres.
The feedback events also temporarily changed the size of the nuclear disk. During one event at approximately 2.8 billion years, for example, its measured radius decreased from around 500 to 400 parsecs before resuming its longer-term growth.
A massive star cluster changes the pattern
The otherwise similar evolution of the nuclear disk and nuclear cluster was interrupted by a major merger. The researchers identified approximately 200 simulated star clusters with masses exceeding 1 lakh solar masses. The largest contained about 3 crore solar masses.
This massive cluster formed farther out in the galaxy and became trapped by the stellar bar. Dynamical friction gradually reduced its orbit, allowing it to enter the nuclear disk and circle the central region several times before merging with the nuclear star cluster at approximately 2.1 billion years.
The merger caused an abrupt increase in the nuclear star cluster's mass and radius. It also produced a star-formation spike within the cluster without creating an equivalent increase across the wider nuclear disk.
This result supports a combined formation picture in which nuclear star clusters can grow through both local star formation and the inward migration of existing star clusters. The importance of each process is likely to vary among galaxies.
Why the connection can be difficult to observe
Observational studies have not found a simple, universal relationship between the masses and sizes of nuclear star clusters and nuclear stellar disks. The simulation offers two possible reasons: the structures continuously change relative to one another, and occasional cluster mergers can rapidly alter the nuclear star cluster without similarly changing the disk.
During the model's secular evolution, the ratio between the cluster and disk radii decreased from about 0.4 to 0.2. Their mass ratio fell from approximately 6 to 2 even though the total mass of the host galaxy remained unchanged.
Consequently, galaxies observed at different evolutionary stages may appear to follow different scaling relationships despite having central structures supplied by a similar underlying process. Accounting for the age of a galaxy's bar and its history of star-cluster accretion could produce clearer comparisons.
Possible implications for the Milky Way
The simulation also provides a model-dependent clue about the Milky Way's central bulge. The simulated galaxy had a bulge-to-disk mass ratio of approximately 0.045, but its nuclear stellar disk was still larger than the Milky Way's observed nuclear disk, whose radius is estimated at roughly 100 parsecs.
The researchers therefore suggest that the Milky Way may possess a less massive classical bulge than the one used in the model. That interpretation could also favour a comparatively younger Galactic bar, because the nuclear disk expands as the bar evolves.
These conclusions remain provisional. The simulation represents an isolated galaxy without external gas accretion, and it does not include magnetic fields, which could modify the history of gas inflow and star formation. It should therefore be treated as a physical model of how galactic centres can evolve, rather than as a complete reconstruction of the Milky Way.
Even with these limitations, the study demonstrates that a nuclear star cluster and nuclear stellar disk can emerge naturally from the same evolving galactic system. Their present-day differences may preserve not only how they formed, but also the age of the stellar bar and the merger history of the galaxy surrounding them.