Hubble Space telescope unveils the first images of ongoing star cluster mergers near the center of dwarf galaxies

A new study reports the first direct observation of merging star clusters in the nuclear region of dwarf galaxies. This detection confirms the feasibility of this formation route for nuclei in dwarf galaxies, which has long been debated. The study was published in Nature science journal, and led by Postdoctoral Researcher Mélina Poulain from the University of Oulu, Finland.

Dwarf galaxies are the most abundant type of galaxies that populate the Universe.  Composed of 100 times fewer stars than the Milky Way, or even less, they are the building blocks of more massive galaxies. 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 such merger of 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 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 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.

The new study is part of Poulain’s research project, funded by the Research Council of Finland. The study was published on 9 April 2025 in the prestigious journal Nature: Reference

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