Dwarf galaxies are the most abundant type of galaxies that populate the Universe. Composed of at least 100 times fewer stars, they are the building blocks of more massive galaxies, such as the Milky Way. Thus, understanding their formation is key to comprehend galaxy evolution.
A notable fraction of dwarf galaxies host a compact star cluster at their centers, typically made of hundreds of thousands to hundreds of millions of stars. Known as nuclear star clusters, these are the densest type of stellar systems in the Universe. The formation of such extreme objects has been under debate for several decades. In dwarf galaxies, they are believed to form from the merger of smaller star clusters, called globular clusters, after they migrate to the galaxy center. However, no merger of such globular clusters has been directly observed to confirm the theory, until now.
Witnessing rare features
While studying observations of a large sample of nearly 80 dwarf galaxies from the Hubble Space telescope, which were led by Francine Marleau at the Department of Astro and Particle Physics, a group of ten researchers from the international MATLAS Collaboration (headed by Pierre-Alain Duc, Strasbourg astronomical observatory, France) 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 University of Innsbruck alumna who led the study. 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 with 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.
Publication: Evidence of star cluster migration and merger in dwarf galaxies. Melina Poulain et.al. Nature 2025 DOI: 10.1038/s41586-025-08783-9