Until recently, the extreme faintness of these structures made them nearly impossible to detect anywhere, let alone in another galaxy. A stream orbiting inside an ultra-diffuse galaxy — itself one of the dimmest galaxy types known — compounds the challenge. Improvements in large astronomical datasets and analysis methods made the new detection possible.

More than a first sighting

The finding matters beyond the novelty of spotting a stream beyond our cosmic doorstep. The researchers also demonstrate, for the first time, that globular cluster stellar streams can be used to measure dark matter in galaxies outside the Milky Way.

"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 Sarah Pearson, who contributed to the discovery during her employment at the Niels Bohr Institute.

Applied to UGC9050-Dw1, the method yielded new estimates of the galaxy's total mass and of how that mass is distributed. The analysis suggests the galaxy holds a large amount of dark matter — consistent with expectations for ultra-diffuse galaxies and with earlier studies of this particular one.

"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.

Caveats and what comes next

The study covers a single galaxy, so the broader power of the method remains to be demonstrated across many systems. Still, the proof of principle is significant: until now, everything astronomers have learned about dark matter from stellar streams has come from one 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.

The researchers expect the number of detectable streams to climb sharply in the coming years. Observatories such as the Euclid Space Telescope and the Nancy Grace Roman Space Telescope are beginning to deliver deep views of faint galaxies and stellar structures that ground-based surveys could not reach.

The study's additional authors are Jacob Nibauer, David J. Sand, Adrian M. Price-Whelan, Tjitske Starkenburg, David Hendel and Catherine Fielder. The Villum Foundation and the European Research Council funded the work.