Evidence of star cluster migration and merger in dwarf galaxies

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
Additional contact information
Mélina Poulain: University of Oulu
Rory Smith: Universidad Técnica Federico Santa María
Pierre-Alain Duc: Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg
Francine R. Marleau: Universität Innsbruck
Rebecca Habas: INAF - Astronomical Observatory of Abruzzo
Patrick R. Durrell: Youngstown State University
Jérémy Fensch: University of Lyon, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon
Sungsoon Lim: Yonsei University
Oliver Müller: École Polytechnique Fédérale de Lausanne (EPFL)
Sanjaya Paudel: Yonsei University
Rubén Sánchez-Janssen: Royal Observatory Edinburgh

Nature, 2025, vol. 640, issue 8060, 902-906

Abstract: Abstract Nuclear star clusters (NSCs) are the densest stellar systems in the Universe. These clusters can be found at the centre of all galaxy types but tend to favour galaxies of intermediate stellar mass around 109M⊙ (refs. 1,2). At present, two main processes are under debate to explain their formation: in situ star formation from gas infall3 and migration and merging of globular clusters (GCs) caused by dynamical friction4. Studies5–9 of NSC stellar populations suggest that the former predominates in massive galaxies, whereas the latter prevails in dwarf galaxies, and both contribute equally at intermediate mass. However, until now, no ongoing merger of GCs has been observed to confirm this scenario. Here we report the serendipitous discovery of five dwarf galaxies with complex nuclear regions, characterized by multiple nuclei and tidal tails, using high-resolution images from the Hubble Space Telescope. These structures have been reproduced in complementary N-body simulations, supporting the interpretation that they result from migrating and merging of star clusters. The small detection rate and short simulated timescales (below 100 Myr) of this process may explain why this has not been observed previously. This study highlights the need for large surveys with high resolution to fully map the migration scenario steps.

Date: 2025
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DOI: 10.1038/s41586-025-08783-9

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