Simulations reveal galactic centers grow together through cosmic gas conveyor

Astronomers have long puzzled over how the dense cores of galaxies take shape—particularly the supermassive black holes and the tightly packed star clusters that orbit them. These structures clearly matter: they influence how stars form, how galaxies evolve, and ultimately how the universe itself has changed since the first galaxies emerged. Yet for decades, a fundamental mystery remained unsolved. Two key features appeared at the centers of most galaxies—nuclear star clusters and nuclear stellar discs—but they seemed to form through separate, unrelated processes. Observations found no clear connection between their masses and sizes. Simulations couldn't reproduce what astronomers actually saw. The pieces didn't fit.

Now, a team led by SungWon Kwak at the Leibniz Institute for Astrophysics Potsdam has cracked open the puzzle using a sophisticated computer model called SMUGGLE-Ring. The breakthrough is elegant: these two structures don't grow independently at all. They grow together, fed by the same source, through a mechanism as simple and powerful as a conveyor belt. The research, published in Astronomy & Astrophysics, involved collaborators from institutions across Europe, the United States, and Asia—from MIT to Tsinghua University to the Observatoire de la Côte d'Azur.

The key player in this cosmic machinery is the galactic bar—a elongated structure of stars that rotates through the center of many spiral galaxies, including our own Milky Way. As the bar rotates, it acts like a funnel, channeling gas inward toward the galactic core. This gas accumulates in the center, where dying stars explode as supernovae, sending shock waves through the surrounding material. These shock waves repeatedly trigger the birth of new stars. Over billions of years, hundreds of millions of solar masses worth of stars form in this crucible. The simulation shows that both the nuclear star cluster and the stellar disc draw from this same reservoir of infalling gas, growing in tandem even as their individual properties diverge.

What makes this finding particularly striking is how it resolves an observational puzzle that had stumped researchers. The two structures appear remarkably different when viewed at different evolutionary stages—different masses, different sizes, different characteristics. This apparent disconnection led astronomers to assume they must be products of fundamentally different processes. But the simulation reveals the truth: they share a common origin and a shared growth mechanism, yet their relative proportions shift over time as they co-evolve. The stars themselves don't differ fundamentally in age or chemical makeup; the structures simply look different depending on when you observe them.

The simulation also uncovered a striking detail with real-world implications. At one point in the model, a star cluster containing 30 million solar masses spiraled inward and merged with the nuclear star cluster, causing it to grow substantially. This isn't mere theoretical speculation. Recent observations of the galaxy NGC 1365 have revealed a massive star cluster embedded in its galactic bar—and astronomers predict this cluster will eventually spiral into the center and merge with NGC 1365's nuclear star cluster, just as the simulation showed. Such mergers could alter not only the size of the nuclear star cluster but potentially the mass of the supermassive black hole itself.

A crucial element of the team's success was their treatment of dark matter. Unlike earlier simulations that used fixed, unchanging backgrounds for the galactic bar and dark matter halo, this model treated dark matter as dynamic—using live particles that interact realistically with stars. This allowed the bar to form naturally and evolve over time, producing structures that matched real observations. The simulation even reproduced a feature called the "dark gap" around the bar region, a phenomenon seen in many actual galaxies and understood as evidence of the interaction between rotating stars and the dark matter halo.

What makes simulations like this invaluable is their ability to compress billions of years into computational time. Researchers can watch a galaxy's stellar bar form from scratch, trace the inward flow of gas, witness the birth of new stars, and observe how the nuclear structures gradually take shape—processes no telescope could ever directly observe. The findings open a new lens for interpreting observations of distant galaxies and for understanding how the Milky Way itself assembled its core. As astronomers continue to gather more detailed observations of galactic centers, these simulations provide the theoretical framework to make sense of what they see.