The Vera C. Rubin Observatory has helped astronomers detect a stellar stream around the barred spiral galaxy M61 (NGC 4303) that is about 10,000 light-years wide and roughly 170,000 light-years long—longer than the visible Milky Way. Researchers interpret the feature as the tidal debris of a dwarf galaxy torn apart by M61’s gravity, with a luminosity near 100 million Suns. The disturbance may be linked to enhanced central activity in M61, and Rubin’s upcoming decade-long LSST survey is expected to reveal many more such faint streams.
Rubin Observatory Reveals Stellar Stream Longer Than the Milky Way Wrapping Galaxy M61
The Vera C. Rubin Observatory has helped astronomers detect a stellar stream around the barred spiral galaxy M61 (NGC 4303) that is about 10,000 light-years wide and roughly 170,000 light-years long—longer than the visible Milky Way. Researchers interpret the feature as the tidal debris of a dwarf galaxy torn apart by M61’s gravity, with a luminosity near 100 million Suns. The disturbance may be linked to enhanced central activity in M61, and Rubin’s upcoming decade-long LSST survey is expected to reveal many more such faint streams.
02:37 AM, 11/21/2025Science
The Vera C. Rubin Observatory, though not yet fully operational, is already revealing previously unseen structures in the nearby universe. Equipped with the 3.2-gigapixel LSSTCam — the largest and most sensitive digital camera in astronomical use — the facility helped astronomers detect a massive stellar stream encircling the barred spiral galaxy Messier 61 (M61, also catalogued as NGC 4303).
The stream is measured at roughly 10,000 light-years across and about 170,000 light-years long, making it longer than the visible width of the Milky Way (about 100,000 light-years). The discovery team interprets the feature as the tidal debris of a dwarf galaxy that was torn apart by M61’s gravity; the shredded stars now form a luminous tidal tail that appears to be perturbing the spiral galaxy’s central region.
M61 lies approximately 55 million light-years from Earth. The team estimates the stream’s luminosity at about 100 million times that of the Sun — surprisingly bright for a structure that went unnoticed around such a well-studied galaxy. As Aaron J. Romanowsky of San José State University notes, revealing these features requires specialized instrumentation and deep imaging techniques.
"The M61 stream is actually relatively bright, and it is surprising that it hadn't been noticed before, around such a nearby, well-known galaxy," said Aaron J. Romanowsky. "I think this highlights the difficulty of detecting even the brighter streams, as special equipment and techniques are needed."
Unlike the Milky Way’s relatively quiet central black hole, M61 hosts an actively accreting supermassive black hole that drives energetic outflows. The researchers suggest that the tidal disruption caused by the infalling dwarf galaxy may be contributing to gas inflows, enhanced star formation, and the current central activity in M61.
Why this matters
This discovery underscores the role of minor mergers and tidal interactions in galaxy growth and evolution. When Rubin begins its decade-long Legacy Survey of Space and Time (LSST), its deep, wide imaging is expected to reveal many more faint stellar streams and a richer low-surface-brightness "hidden universe" than previously known.
The team's results are available on the preprint server arXiv, and the find invites follow-up observations as Rubin approaches full operations.