A Hidden River of Stars: Discovering a Stellar Stream Around Galaxy M61 with the Rubin Observatory
The newly operational Vera C. Rubin Observatory has already made an exciting discovery: a faint, narrow river of stars, known as a stellar stream, flowing around the spiral galaxy Messier 61 (M61). In this study, led by Aaron Romanowsky, astronomers present the first stellar stream detected in Rubin’s “First Look” imaging of the Virgo Cluster. The stream extends about 50,000 light-years (50 kpc) from M61, and its discovery hints at the powerful role of galaxy mergers in shaping the cosmos.
Galactic Cannibalism and Cosmic Recycling
Massive spiral galaxies, including our own Milky Way, grow by swallowing smaller galaxies. As these dwarf galaxies are torn apart by gravity, they leave behind long trails of stars called stellar streams. These structures are valuable to astronomers because they preserve evidence of how galaxies form and evolve, and can even reveal details about dark matter, the invisible substance thought to make up most of the universe. Romanowsky and his team point out that while astronomers have previously studied stellar streams around the Milky Way and nearby galaxies, the Rubin Observatory’s unprecedented sensitivity now allows such discoveries in more distant systems.
The Discovery of the M61 Stream
During Rubin’s early commissioning phase in 2025, the observatory imaged 25 square degrees of the Virgo Cluster using multiple filters. The images reached a depth nearly equivalent to what is expected after five years of the full Legacy Survey of Space and Time (LSST). While examining these early datasets, astronomer Giuseppe Donatiello noticed an intriguing linear feature extending northward from M61, a galaxy known for its bright spiral arms and a central starburst region. Upon further study, this feature turned out to be a remarkably straight, thin stellar stream beginning about 20 kpc from the galaxy’s center and stretching for over twice that distance.
Characterizing the Stream
The team used imaging data from both Rubin and the DECaLS survey to measure the brightness and color of the stream. They found that its light gradually fades from a surface brightness of 27.2 to 28.6 magnitudes per square arcsecond, making it extremely faint, barely visible even in deep survey images. Its color, with a measured g – z value of about 1.0, suggests that the stream’s stars are old and no longer forming new ones, similar to those found in small, inactive dwarf galaxies. The total mass of the stream is estimated to be around 200 million times the mass of the Sun, comparable to the Sagittarius stream currently orbiting the Milky Way.
What the Stream Reveals About M61
The team proposes that the stream is the remnant of a dwarf galaxy being torn apart by M61’s gravity. This merger could have triggered several of M61’s striking features, including the formation of its central bar, an ongoing starburst near the nucleus, and even its active galactic nucleus (AGN). If true, M61 would be an example of how galaxy interactions can drive dramatic internal changes. The authors note that the stream’s narrowness and relatively short arc suggest it represents an early stage of disruption, essentially catching the process of galactic cannibalism in action.
A Glimpse of the Future
Romanowsky and collaborators emphasize that this discovery is just the beginning. The detection of a structure this faint and detailed in early test images demonstrates the incredible power of the Rubin Observatory for mapping faint galactic features. As the LSST progresses over the next decade, it is expected to uncover thousands of similar structures, painting a much clearer picture of how galaxies build themselves over cosmic time. What went unnoticed for centuries around one of the Messier catalog’s famous galaxies may now be the first glimpse of a new era of galactic archaeology.
Source: Romanowsky
