Evrim Yazgin
Cosmos science journalist
NASA’s Hubble Space Telescope continues to break new scientific ground despite being usurped by the more advanced James Webb Space Telescope.
A new study published in Nature reveals images from Hubble which are the first to show ongoing mergers of star clusters near the centres, or nuclei, of dwarf galaxies.
Dwarf galaxies are the most common galaxy type in the universe. They have on average hundreds of thousands up to billions of stars. For comparison, the Milky Way is estimated to have 100 to 400 billion stars.
Cosmologists believe that dwarf galaxies are the building blocks of larger galaxies.
At their centres are compact star clusters which make up the bulk of the dwarf galaxy’s mass. These nuclear star clusters are the densest grouping of stars anywhere in the universe.
How these clusters formed has been a mystery. Cosmologists have suggested that they form from the merger of smaller globular clusters. Such mergers have not been observed until now.
Researchers belonging to the international collaboration MATLAS – a deep imaging survey of massive galaxies and their surroundings – were studying Hubble images of nearly 80 dwarf galaxies when they noticed a handful of them had unusual looking nuclear star clusters.
Some appeared to have star clusters close together, while others had faint streams of light attached to the nuclear star cluster.
“We were surprised by the streams of light that were visible near the centre of the galaxies, as nothing similar has been observed in the past,” explains first author of the Nature paper Mélina Poulain, from the University of Oulu, Finland.
Further analysis showed that what the astronomers were looking at was the dramatic merging of globular clusters at the dwarf galaxies’ cores to form nuclear star clusters.
Computer simulations of the merger of globular clusters gave even more insight into the features observed in the Hubble images.
The streams of light seen in some galaxies emerge when there is a collision between 2 globular clusters which are significantly different in size. The greater the ratio between their sizes, the longer the stream of light.
These mergers usually last less than 100 million years. And the features produced are visible for even less time. This helps explain why it has been so difficult to catch the formation of nuclear star clusters until now.
