Astronomers have, for the first time, directly observed star clusters merging at the cores of dwarf galaxies—a process long theorized but never confirmed.
This cosmic cannibalism, seen via Hubble and simulated in high detail, helps solve the mystery of how ultra-dense nuclear star clusters form in the Universe’s smallest galaxies. It’s a rare and fleeting event, offering a new piece of the puzzle in galaxy evolution.
First Glimpse of Star Cluster Mergers
For the first time, astronomers have directly observed star clusters merging at the centers of dwarf galaxies—offering long-awaited evidence for a widely debated theory. The discovery, published in Nature and led by postdoctoral researcher Mélina Poulain from the University of Oulu in Finland, confirms that dense star clusters at the hearts of dwarf galaxies can form through the merging of smaller clusters.
Dwarf galaxies are the most common type of galaxy in the Universe. They contain far fewer stars than galaxies like the Milky Way, sometimes just one-hundredth as many, and serve as the fundamental building blocks of larger galaxies. Understanding how dwarf galaxies evolve is essential to piecing together the broader story of galaxy formation.
Mystery of Nuclear Star Clusters
Many dwarf galaxies host a tightly packed group of stars at their center, known as a nuclear star cluster. These are some of the densest stellar systems in the cosmos, containing hundreds of thousands to millions of stars. For decades, scientists have debated how these extreme clusters form. One leading theory suggests that smaller star clusters, known as globular clusters, slowly migrate to the galaxy’s core and eventually merge. Until now, however, no one had caught this process in action.
Witnessing Rare Features
While studying observations of a large sample of nearly 80 dwarf galaxies from the Hubble Space telescope, which were led by Prof. Francine Marleau at the University of Innsbruck, Austria, a group of ten researchers from the international MATLAS collaboration noticed a handful of galaxies with an unusual looking nuclear star cluster. Some showed a couple of star clusters close together, while others had a feature similar to a faint stream of light attached to the nuclear star cluster.
“We were surprised by the streams of light that were visible near the center of the galaxies, as nothing similar has been observed in the past,” explains Mélina Poulain.
A thorough analysis of the features has shown that they have similar properties to globular clusters already detected in dwarf galaxies. This suggests that the observations witness the growth of the nuclear star cluster by the dramatic cannibalization of globular clusters at the cores of those galaxies.
Observations Reproduced in Simulations
To confirm the origin of the faint streams of light, ultra-high resolution complementary simulations were implemented to model the merging process. This portion of the work, led by Dr. Rory Smith at the Universidad Técnica Federico Santa María in Santiago, Chile, set up various mergers between star clusters with differing masses, dynamics, and numbers of clusters involved.
Results confirm that the observed light streams are created when two star clusters with significant mass differences merge. The larger the mass ratio, the longer the stream. The process typically lasts a short amount of time, less than 100 million years, and the features produced are visible for even less time, which explains the difficulty of catching such a phenomenon.
Reference: “Evidence of star cluster migration and merger in dwarf galaxies” by Mélina Poulain, Rory Smith, Pierre-Alain Duc, Francine R. Marleau, Rebecca Habas, Patrick R. Durrell, Jérémy Fensch, Sungsoon Lim, Oliver Müller, Sanjaya Paudel and Rubén Sánchez-Janssen, 9 April 2025, Nature.
DOI: 10.1038/s41586-025-08783-9
The new study is part of Poulain’s research project, funded by the Research Council of Finland.
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