First stellar stream beyond the Milky Way could reveal dark matter

Because these stellar streams respond to gravity, they can act as tracers of dark matter that cannot be seen directly. Until recently, their extreme faintness made them very difficult to detect and analyze. Improvements in massive astronomical datasets and advanced analysis methods are now turning stellar streams into one of the most promising tools for studying galaxies.

First Stellar Stream Found 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 now reported a first-of-its-kind discovery. The results were published in Nature, one of the world's most prestigious scientific journals.

"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.

Finding this kind of structure beyond the Milky Way is especially difficult because the stellar stream itself produces such a weak signal. The newly detected stream is also located inside an ultra-diffuse galaxy, a type of galaxy that gives off very little light.

That combination makes the detection particularly striking.

"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.

What is a globular cluster stellar stream?

A globular cluster stellar stream is a long, narrow trail of stars that originated in a globular cluster, a dense collection of stars held together by gravity.

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

Because gravity determines the shape and motion of stellar streams, astronomers can study them to learn how mass is distributed throughout a galaxy. That makes the streams useful for investigating dark matter, which represents approximately 80-85 percent of all matter in the universe even though its fundamental nature remains unknown.

A New Way to Map Dark Matter

The importance of the finding goes beyond simply identifying a stellar stream in another galaxy. The researchers also show for the first time that globular cluster stellar streams can be used to measure dark matter in galaxies outside the Milky Way.

That ability could be valuable because dark matter accounts for most of the Universe's mass, yet scientists still do not know what it is made of.

"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 Julie Kiel Holm.

The team focused on the ultra-diffuse galaxy UGC9050-Dw1 and produced new estimates of both its total mass and how that mass is distributed. Their analysis suggests that the galaxy contains a large amount of dark matter, consistent with expectations 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 Julie Kiel Holm.

Extending Dark Matter Studies to Other Galaxies

Although the research centers on just one galaxy, the method could eventually have much wider applications. Demonstrating that globular cluster stellar streams can be studied beyond the Milky Way could allow astronomers to investigate dark matter in many different types of galaxies.

"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 Julie Kiel Holm.

Researchers expect the number of detectable globular cluster stellar streams to rise dramatically as observatories such as the Euclid Space Telescope and the Nancy Grace Roman Space Telescope provide new views of faint galaxies and stellar structures.

About the study

• The study reports the first detection of a globular cluster stellar stream outside the Milky Way.
• The researchers also show that these streams can be used to constrain both the dark matter halo mass and the dark matter density profile of galaxies beyond our own.
• The study has been published in Nature.
• The following authors contributed to the study: Julie Kiel Holm, Sarah Pearson, Jacob Nibauer, David J. Sand, Adrian M. Price-Whelan, Tjitske Starkenburg, David Hendel and Catherine Fielder.
• The study is funded by the Villum Foundation and the European Research Council.