Stellar stream beyond Milky Way offers new tool to map dark matter

In the image above, a faint trail of stars can be seen stretching across the galaxy. These structures, known as globular cluster stellar streams, are coherent stellar structures that retain a record of their dynamical history and can provide unique insights into the evolution of galaxies and the nature of dark matter.

Globular cluster stellar streams offer astronomers a unique opportunity to map otherwise invisible dark matter and study how it behaves. For many years, they have been difficult to observe because they are extremely faint. However, advances in large astronomical data sets and sophisticated analysis techniques have recently made stellar streams one of the most promising tools in galactic astronomy.

Now, Ph.D. student Julie Kiel Holm from the Niels Bohr Institute and associate professor Sarah Pearson from DTU Space, together with an international team of researchers, have made a discovery that has never been seen before. Their findings have just been published in Nature.

“We have discovered a globular cluster stellar stream in another galaxy. This is the first time such a stream has been observed outside our own galaxy, the Milky Way, which makes the discovery particularly exciting,” says Julie Kiel Holm.

Globular cluster stellar streams are extremely difficult to detect beyond the Milky Way because their signal is so faint. In this case, the stream was identified in an ultra-diffuse galaxy, a class of galaxies that emit very little light.

It is precisely the combination of these two factors that makes the discovery so remarkable.

“This opens entirely new possibilities. Not only can we now search for globular cluster stellar streams in other galaxies, but in the long term, we may also be able to measure the dark matter content of more ultra-diffuse galaxies,” says co-author Sarah Pearson, who contributed to the discovery during her employment at the Niels Bohr Institute.

A new tool for mapping dark matter

The significance of the discovery lies not only in the detection itself. The researchers also demonstrate, for the first time, that globular cluster stellar streams can be used as a tool for measuring dark matter in galaxies beyond the Milky Way. This is important because dark matter makes up most of the mass in the universe, yet its nature remains unknown.

“We show that a well-established tool from studies of the Milky Way can be used to understand other galaxies, where measuring the distribution of dark matter has traditionally been very challenging,” says Holm.

The researchers studied the ultra-diffuse galaxy UGC9050-Dw1 and present new estimates of how its mass is distributed and how much mass it contains. Their analyses indicate that the galaxy contains large amounts of dark matter, as expected for ultra-diffuse galaxies.

“Our results are consistent with previous studies and what they have shown about dark matter in this ultra-diffuse galaxy. We are measuring it with a completely new tool, demonstrating that this method also works beyond our own galaxy,” says Holm.

A single discovery could help explain the universe

Although the study focuses on a single galaxy, its broader significance lies in the possibility of gaining a deeper understanding of dark matter itself. By establishing globular cluster stellar streams as a practical tool beyond the Milky Way, researchers may eventually be able to map dark matter across a wide range of galaxy types.

“The insights into dark matter that we have previously been able to gain from globular cluster stellar streams have been limited to a single galaxy—our own. Being able to observe these streams in entirely different kinds of galaxies opens the door to using them to build a much broader understanding of how dark matter behaves,” says Holm.

With new and upcoming facilities such as the Euclid Space Telescope and the Nancy Grace Roman Space Telescope, the researchers expect the number of observable globular cluster stellar streams to increase dramatically.

The following authors contributed to the study: Holm, Pearson, Jacob Nibauer, David J. Sand, Adrian M. Price-Whelan, Tjitske Starkenburg, David Hendel and Catherine Fielder.

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