Using a state-of-the-art galaxy simulation, researchers at the Leibniz Institute for Astrophysics Potsdam (AIP) and international partners have discovered that nuclear star clusters and nuclear stellar discs in galactic centers grow together rather than forming independently, according to a study published in Astronomy & Astrophysics. The findings resolve a long-standing astrophysics puzzle regarding the formation mechanics of supermassive black holes and their surrounding galactic cores, helping scientists track how the Universe has evolved since the first galaxies emerged.
How Cosmic Conveyor Belts Build Galactic Cores
For roughly a decade, astronomers have observed nuclear star clusters and nuclear stellar discs surrounding supermassive black holes in the Milky Way and other galaxies. Observational surveys previously found no clear correlation between their masses and sizes. Because of this, scientists long assumed these structures were products of separate formation processes.
To investigate this, a research team utilized the Stellar Feedback in Galaxies and its Effects, or SMUGGLE-Ring, project. This advanced physics framework models how stars and stellar feedback shape galaxies. Through high-resolution hydrodynamical simulations, the team demonstrated that a barred galaxy like the Milky Way can naturally form both structures together over billions of years, according to lead researcher SungWon Kwak, a postdoctoral researcher from AIP.
The simulation reveals that a galaxy’s stellar bar acts as a cosmic conveyor belt. It funnels gas inward toward the center from the exact same reservoir. As gas accumulates, supernovae from dying stars generate shock waves that repeatedly trigger the formation of new stars. Over billions of years, hundreds of millions of solar masses’ worth of stars form in the galactic center.
Did you know?
An enormous concentration of light in a compact region produces the diffraction spike seen at the center of NGC 1365, which is actually a telescope artifact.
Why Previous Observations Missed the Connection
The new simulation results explain why earlier observational surveys failed to find a clear connection between nuclear star clusters and nuclear stellar disks. According to Dr. Cristina Chiappini, a scientist from AIP and co-author of the study, the apparent disconnection does not mean that the stars themselves differ fundamentally in age, chemical composition, or motion.
Furthermore, the simulation showed that the structural relationship between the two processes naturally evolves over time. Throughout phases of continuous development, the disk and cluster begin to exhibit distinct differences in their respective sizes and masses. Despite sharing an underlying growth mechanism, the clusters and stellar disks look remarkably different when observed at various stages in their evolutionary lifecycle.
The Role of Infalling Star Clusters
The dynamics of galactic co-evolution prove even more complex than steady gas funnels. At one point in the simulation, a star cluster weighing 30 million solar masses spiraled into the galactic center and merged directly with the nuclear star cluster.
This simulation outcome mirrors recent real-world observations of the galaxy NGC 1365, which revealed a massive star cluster inside its bar. Astronomers expect that cluster to eventually spiral into the galaxy’s center and merge with its nuclear star cluster, increasing its overall size and mass. Researchers note this ongoing process could also impact the mass of the supermassive black hole housed inside the nuclear star cluster over time.
Advantages of Advanced Astrophysical Simulations
Observing these deep-space mechanics directly is impossible with current telescopes because these processes unfold over billions of years. Researchers can rely on simulations to observe how a galaxy’s stellar bar forms, follow the inward migration of gas, track the generation of newborn stars, and view the outward expansion of the nuclear stellar disk from the center.
Dark matter also plays a crucial role in shaping these results. As co-author Dr. Ivan Minchev pointed out, these findings expand our understanding of how galactic components connect, helping astronomers interpret future observations.
Led by Ivan Minchev from AIP, the international research team also featured investigators from the Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, the Università di Bologna, the Observatoire de la Côte d’Azur (OCA), Tsinghua University, the University of California, Riverside, the Kavli Institute for Astrophysics and Space Research at MIT, the SNU Astronomy Research Center, and the Institut für Physik und Astronomie at the Universität Potsdam.
Frequently Asked Questions
What are nuclear star clusters and nuclear stellar discs?
They are densely packed stellar structures that surround supermassive black holes at the centers of most galaxies in the Universe, including the Milky Way.
Why did astronomers previously think these structures formed independently?
Observational surveys found no clear correlation between the masses and sizes of nuclear star clusters and nuclear stellar discs, and older simulations could not realistically reproduce their concurrent development.
What role does the galactic bar play in galaxy evolution?
According to AIP researchers, a galaxy’s stellar bar acts like a cosmic conveyor belt, channeling gas inward toward the center to feed both the nuclear star cluster and the nuclear stellar disc simultaneously.
Can astronomers observe these galaxy formation processes directly?
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