The Vera Rubin Observatory (VRO) has made a significant discovery by identifying a massive stellar stream surrounding the galaxy Messier 61 (NGC 4303). This finding emerged from its preliminary observations, known as the Virgo First Look images, taken when the observatory saw its first light in June 2025.
Although the VRO has yet to commence its much-anticipated Legacy Survey of Space and Time, the initial images demonstrate its capabilities. These images showcase the Virgo Cluster, a well-studied group of galaxies, but the VRO”s advanced technology has revealed new and unexpected features.
A research letter titled “A stellar stream around the spiral galaxy Messier 61 in Rubin First Look imaging” details the discovery. It is set to be published in the Research Notes of the American Astronomical Society, with Aaron Romanowsky from San Jose State University as the lead author. The authors highlight that one particularly striking feature in the VRO images is a long, narrow stellar stream extending northward from Messier 61.
Stellar streams like this are typically remnants of smaller galaxies or globular clusters that have been disrupted by the gravitational forces of larger galaxies. The stream detected around M61 is notably extensive, measuring approximately 163,000 light years in length, dwarfing similar streams found within the Milky Way, which are usually tens of thousands of light years long.
According to the authors, giant spiral galaxies like Messier 61 and the Milky Way are in a constant process of accreting smaller dwarf galaxies, which create these stellar streams as a result of tidal disruptions. The stream observed around M61 may be linked to a starburst event in the galaxy”s core that occurred roughly 10 million years ago.
Furthermore, the progenitor galaxy of this newly discovered stream may have influenced M61″s structure in more profound ways. The authors note that the progenitor could be responsible for various features observed in M61, including its bar formation, starburst activity, and its active galactic nucleus, drawing a parallel to the impact of the Sagittarius Dwarf Spheroidal Galaxy on the Milky Way.
The stream also possesses a complex end structure, measuring about 9 x 4 kiloparsecs or roughly 30,000 by 13,000 light years, which is expected to be the subject of further detailed analysis.
This exciting discovery has heightened anticipation for the VRO”s future observations. The authors conclude that it is remarkable this stellar stream had remained unnoticed around a well-studied Messier galaxy. They expect the VRO”s continued observations to uncover a wealth of substructures surrounding numerous other galaxies.
For those interested, an interactive version of some early imagery from the VRO is available for exploration.
