The Vera Rubin Observatory’s first-light images from June 2025 reveal a massive stellar stream around Messier 61 (NGC 4303) in the Virgo Cluster, detected during commissioning.

The stream extends roughly 163,000 light-years northward from M61, far longer than typical Milky Way streams, and appears to be the remnant of a dwarf galaxy or globular cluster torn apart by M61’s tides.

Rubin First Look imaging identifies an enormous northward-extending stellar stream, about 163,000 light-years in length.

An interactive image and additional details are available through Rubin Observatory sources and related science notes detailing the findings and their implications.

The discovery echoes parallels with the Sagittarius Dwarf Stream around the Milky Way, which has driven star formation episodes in our galaxy.

Authors suggest this stream is part of a broader population of substructures revealed by Rubin data, signaling many more discoveries as the LSST program begins.

Rubin data are expected to reveal numerous substructures around other galaxies, underscoring the telescope’s observational potential in early commissioning.

The stream ends in a complex plume spanning about 9 by 4 kiloparsecs (roughly 30,000 by 13,000 light-years), warranting further study.

The stream’s progenitor may have influenced M61’s evolution, potentially driving bar formation, a recent central starburst, and a possible active galactic nucleus, mirroring effects seen with the Sagittarius Dwarf.

The progenitor is hypothesized to be an infalling dwarf with an estimated halo mass around 8 × 10^10 solar masses, capable of imprinting onto M61’s bar, starburst activity, and AGN.

An arXiv preprint detailing the finding is available, led by Aaron J. Romanowsky of San Jose State University.

The discovery underscores how stellar streams trace galaxy formation and dark matter, illustrating hierarchical accretion’s role in shaping large spiral galaxies like M61.