A recent research presents the initial direct evidence of colliding star clusters within the central area of dwarf galaxies. This finding validates the possibility of this development pathway for cores in dwarf galaxies, an idea that has been contested for a long time. The investigation was published in Nature , with leadership provided by Postdoctoral Researcher Mélina Poulain from the University of Oulu, Finland.
The most prevalent galaxies in the cosmos are dwarf galaxies. These galactic entities contain as little as one-hundredth the number of stars found within the Milky Way or perhaps an even lesser amount. They serve as fundamental components for larger galaxies. Consequently, grasping how these dwarfs form is crucial for unraveling the processes behind galaxy development.
A significant portion of dwarf galaxies contain a compact star cluster at their cores, usually comprising from hundreds of thousands to upwards of hundreds of millions of stars. These are referred to as nuclear star clusters and represent the most densely packed groupings of stars known in the cosmos. The creation of these extreme celestial bodies has sparked considerable discussion among scientists over multiple decades.
In dwarf galaxies, it is thought that these structures originate from the amalgamation of tinier star groupings known as globular clusters once they move towards the galactic core. Nevertheless, up till recently, no direct observation of such a merging event involving globular clusters had taken place to substantiate this hypothesis.
Witnessing rare features
During their examination of observations from a substantial collection of almost 80 dwarf galaxies using the Hubble Space Telescope, a team of ten scientists under the leadership of Professor Francine Marleau at the University of Innsbruck in Austria made significant findings. 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 astonished by the beams of light observable at the galactic centers, since nothing akin to this has ever been seen before," clarifies Poulain.
An extensive examination of these characteristics indicates that they closely resemble the attributes of globular clusters previously identified within dwarf galaxies. This implies that the observed phenomena may be indicative of the expansion of the central star cluster through the significant assimilation of globular clusters located at the centers of these galaxies.
Observations reproduced in simulations
To verify the source of the dim beams of light, highly detailed complementary simulations were conducted to depict the merger events. Led by Dr. Rory Smith from the Universidad Técnica Federico Santa MarÃa in Santiago, Chile, this segment of the research examined multiple collisions involving star clusters with varying masses, dynamics, and quantities of participating clusters.
The findings show that the detected light streaks arise when two star clusters with notable mass disparities merge. A greater mass ratio leads to extended streams. This event usually occurs over a brief period, under 100 million years, and the resulting structures become observable for an even shorter duration, which accounts for the challenge in observing this occurrence.
More information: Melina Poulain, Proof of star cluster migration and merging within dwarf galaxies, Nature (2025). DOI: 10.1038/s41586-025-08783-9 . www.nature.com/articles/s41586-025-08783-9
Furnished by the University of Oulu
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