Because stellar streams respond to gravity, they can act as trackers of dark matter, which cannot be observed directly. Until recently, these streams were extremely difficult to detect and analyze because they produce such a weak signal. Now, vast astronomical datasets and advances in analytical methods are making star streams one of the most promising tools for studying galaxies.

First stellar stream discovered beyond the Milky Way

An international team led by PhD student Julie Kiel-Holm of the Niels Bohr Institute and Associate Professor Sarah Pearson of DTU Space has reported the first discovery of a globular cluster star stream outside the Milky Way. The study was published in Nature, one of the world’s most prestigious scientific journals.

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

Detecting this type of structure is difficult even within the Milky Way because a star stream produces a very weak signal. The newly detected stream is also located inside a superdiffuse galaxy, a type of galaxy that emits relatively little light.

This combination makes the discovery especially significant.

“This opens up entirely new possibilities. Not only can we now look for star streams in globular clusters in other galaxies, but in the long term we may also be able to measure the dark matter content of more hyperdiffuse galaxies,” says co-author Sarah Pearson, who contributed to the discovery while working at the Niels Bohr Institute.

What is a globular cluster star stream?

Globular cluster star streams are elongated trails of stars that originate from globular clusters—dense groups of stars held together by gravity.

These streams form when the gravity of a surrounding host galaxy gradually pulls stars away from a globular cluster through a process called tidal detachment.

Gravity determines the shape and motion of star streams, allowing astronomers to study how mass is distributed across galaxies. This makes the newly observed stream a potential tool for investigating dark matter, which accounts for about 80 to 85 percent of all matter in the universe, although its fundamental properties remain unknown.

A new way to map dark matter

The significance of the discovery goes beyond identifying a star stream in another galaxy. For the first time, the researchers showed that a globular cluster star stream can be used to measure dark matter in galaxies beyond the Milky Way.

This capability could be valuable because dark matter makes up most of the mass in the universe, yet scientists still do not know what it is made of.

“We have shown that tools established in the study of the Milky Way can be used to understand other galaxies, where it has traditionally been very difficult to measure the distribution of dark matter,” says Julie Kiel-Holm.

The research team focused on the superdiffuse galaxy UGC9050-Dw1 and produced new estimates of its total mass and how that mass is distributed. Their analysis suggests that the galaxy contains large amounts of dark matter, consistent with expectations for superdiffuse galaxies.

“Our results are consistent with previous studies and what has been shown about dark matter in this superdiffuse galaxy. We are measuring it with a completely new tool and demonstrating that this method works beyond our galaxy,” says Julie Kiel-Holm.

Extending dark matter research to other galaxies

Although the study focuses on a single galaxy, the technique could eventually be applied more broadly. Showing that astronomers can study globular cluster star streams beyond the Milky Way may make it possible to investigate dark matter in many different types of galaxies.

“Until now, the insights into dark matter that we have been able to gain from star streams in globular clusters have been limited to a single galaxy, our own. Being able to observe these streams in a completely different type of galaxy opens the door to using them to build a broader understanding of how dark matter behaves,” says Julie Kiel-Holm.

Researchers expect the number of detectable globular cluster star streams to increase dramatically as observatories such as the Euclid Space Telescope and the Nancy Grace Roman Space Telescope provide new perspectives on faint galaxies and their stars.

About the research

• The study reports the first detection of a globular cluster star stream outside the Milky Way.
• The researchers show that these streams can help constrain both the dark matter halo mass and the dark matter density profile of galaxies beyond our own.
• The study was published in Nature.
• Authors contributing to the study are Julie Kiel-Holm, Sarah Pearson, Jacob Nibauer, David J. Sand, Adrian M. Price-Whelan, Tjitske Starkenburg, David Hendel, and Catherine Fielder.
• The research was funded by the Villum Foundation and the European Research Council.