Distant stellar stream offers new way to map dark matter

Astronomers have identified a faint trail of stars in a galaxy beyond the Milky Way, marking the first detection of a globular cluster stellar stream outside our own galaxy. The observation also demonstrates that these delicate structures can be used to investigate dark matter in distant galaxies.

The international research team was led by Julie Kiel Holm of the Niels Bohr Institute and Sarah Pearson of DTU Space. Its findings were published in Nature, according to ScienceDaily, which reported on material supplied by the University of Copenhagen.

The stream lies in UGC9050-Dw1, an ultra-diffuse galaxy. Such galaxies emit very little light, making both the galaxy and the narrow stellar feature inside it difficult to study. The researchers used the newly identified stream to produce estimates of the galaxy’s overall mass and the way that mass is spread through it.

Their results indicate that UGC9050-Dw1 contains a substantial quantity of dark matter, in line with earlier work on ultra-diffuse galaxies. The significance of the study is not only the discovery of the stream, but the demonstration that a technique previously used in the Milky Way can work in another galaxy.

A trail shaped by gravity

Globular clusters are dense groups of stars bound together by their mutual gravity. As a cluster travels through its host galaxy, the larger galaxy’s gravitational field can gradually pull stars away. Over time, those stars form a long, thin stream along the cluster’s path.

The structure of a stream reflects the gravitational forces acting on it. By examining its shape and movement, astronomers can infer how matter is distributed in the surrounding galaxy. This makes stellar streams useful indirect markers of dark matter, which cannot be observed directly.

Dark matter is thought to make up roughly 80 to 85 percent of all matter in the universe, although its underlying nature remains unknown. Its presence is inferred through gravity, including the effects it has on stars and other visible material. Stellar streams provide another way to trace those effects because their development depends on the gravitational environment around them.

Until recently, streams were difficult to find because they are extraordinarily dim. More extensive astronomical surveys, larger collections of observations and improved analytical techniques are now making it possible to detect and study features that would previously have gone unnoticed.

Extending a Milky Way method

Before this result, the use of globular cluster streams to study dark matter had been confined to the Milky Way. That limitation meant astronomers had information from only one galaxy when considering how such structures could reveal the distribution of unseen mass.

The observation in UGC9050-Dw1 expands the method to a different galactic setting. Ultra-diffuse galaxies are especially valuable targets because their faint appearance raises questions about how much dark matter they contain and how that matter is arranged. The new analysis provides a separate route to addressing those questions.

Holm said the work shows that a tool established through studies of the Milky Way can be applied to galaxies where dark matter has traditionally been much harder to measure. Pearson said the result creates the possibility of searching for similar streams in other galaxies and, eventually, assessing the dark matter content of a larger population of ultra-diffuse systems.

The researchers stress that the current study concerns one galaxy. Even so, confirming that the method functions beyond the Milky Way could allow comparisons among galaxies with different properties. Those comparisons may help astronomers develop a broader picture of how dark matter is distributed and behaves across the universe.

More discoveries ahead

The number of known examples could grow as new observatories examine faint galaxies and their surrounding structures. The Euclid Space Telescope and the Nancy Grace Roman Space Telescope are expected to provide views that may help reveal additional stellar streams.

A larger sample would give researchers more opportunities to test the relationship between stream features, galaxy structure and dark matter. For now, the observation of the stream in UGC9050-Dw1 establishes that one of the Milky Way’s most useful gravitational tracers can also be applied at greater distances and in a markedly different type of galaxy.