A thin trail of stars has been spotted in a galaxy beyond the Milky Way. It appears to have originated from a globular cluster, a very dense group of old stars. This would be the first structure of its kind observed in another galaxy, with a particular significance: it could help measure its dark matter.
The galaxy in question is called UGC9050-Dw1. Astronomers identified a narrow, elongated band of stars there using deep images from the Hubble Space Telescope.
Visualization of models that may correspond (left) or not (right) to the observations, according to different scenarios involving dark matter.
This trail could be the remnant of a globular cluster gradually distorted by the gravity of its galaxy. Stars may have been stripped from the group as it moved, then stretched along its orbit.
This type of structure is called a stellar stream. Several examples are already known in the Milky Way. Their shape depends directly on the gravity encountered by the stars along their path.
That is precisely what makes this discovery interesting for the study of dark matter. Dark matter does not produce light, but its mass alters the movements of stars. The shape of the stream can therefore serve as an indirect trace of its presence.
The researchers compared the observed structure with simulations. Its appearance, color, and the presence of a nearby compact object match what is expected from an old globular cluster undergoing disruption.
Their models also indicate that UGC9050-Dw1 could contain a significant amount of dark matter. This estimate remains dependent on the scenario chosen to explain the trail and will need to be refined with new observations.
The significance extends beyond this single galaxy. If other similar streams are detected elsewhere, astronomers will be able to compare the distribution of dark matter in several galaxies. Until now, this method had mainly been used within the Milky Way.