Researchers using Rubin preview data find two hidden Milky Way companions

Researchers using preview data from the recently completed NSF-DOE Vera C. Rubin Observatory in Chile have identified two unknown companions of the Milky Way. The findings come from Early Data Preview 2, observations collected between April 2025 and January 2026 that covered about 7% of the night sky. The preview data came from observations taken before the observatory’s official survey launch.

One companion is a galaxy named Aquarius IV in the Milky Way’s distant outskirts, roughly 355,000 light-years from Earth. William Cerny, a Yale astronomy graduate student, led the Aquarius IV study, which appeared in Research Notes of the AAS. Cerny is second author on the companion paper.

The second companion is Rubin-GC1, described as a dust-shrouded halo globular cluster in Ophiuchus on the far side of the Milky Way. Thick cosmic dust in that murky region helped keep it hidden. Aashay Pai of the University of Chicago is first author on the Rubin-GC1 study, accepted by The Astrophysical Journal Letters and available as a preprint on arXiv.

Astronomers know of about 65 satellite galaxies of the Milky Way, while speculation holds that hundreds more remain undetected. A galaxy satellite is a small galaxy that orbits a larger one. Ultra-faint galaxies like Aquarius IV contain a fraction of the stars found in larger galaxies and are almost entirely composed of dark matter. These dim systems offer clues to the nature of dark matter itself.

The observatory’s 8.4-meter Simonyi Survey Telescope and its 3.2-gigapixel camera, the largest digital camera ever built, image the Southern Hemisphere sky. The instrument is building an ultra-wide, time-lapse record of the night sky. Rubin began its 10-year Legacy Survey of Space and Time in June. A central goal of that survey is to map the Milky Way’s halo—the sphere of gas, stars and dark matter surrounding the galaxy—and find hidden satellite galaxies, star clusters and other substructures.

The survey will image the southern sky every three to four nights, photographing the entire visible sky about 800 times over ten years. Those repeated revisits allow scientists to detect anything that changes, from exploding stars to moving asteroids. The catalog will include approximately 20 billion galaxies, 17 billion stars within the Milky Way and six million small solar system bodies. That census will be far larger than previous surveys.

William Cerny said, “its full 10-year survey will undoubtedly provide us with many more Milky Way satellite discoveries,” because preview data already revealed systems this faint and difficult. He added that the new satellites give researchers opportunities to test models of galaxy formation, trace the Milky Way’s assembly history and investigate the nature of dark matter.

Marla Geha, a Yale professor of astronomy and physics and co-author on both studies, called the discoveries incredibly promising. “Continued discoveries will allow us to complete the census of satellite galaxies around the Milky Way,” she said. She said completing the census is key to understanding how the faintest galaxies form and described finding new Milky Way satellites in Rubin data as a career-long prospect that is now a reality.

The researchers describe dark matter as the invisible scaffolding that makes up most of the matter in the universe. Vera Rubin’s 1970s observations of flat rotation curves showed stars at galaxy edges move as quickly as stars near centers, indicating galaxies hold five to ten times more mass than visible matter. Dark matter was first proposed by Fritz Zwicky in the 1930s.

Ordinary matter accounts for just 5 percent of the universe, with dark energy comprising 68 percent and dark matter 27 percent; the observatory is named for Rubin because her work gave compelling evidence for dark matter, and it is funded jointly by the National Science Foundation and the U.S. Department of Energy’s Office of Science.

Rubin’s first imagery, released on June 23, 2025, showed 10 million galaxies in a single composite image—about 0.05% of the roughly 20 billion galaxies the observatory will photograph over the decade. The release included views of the Trifid and Lagoon nebulae and a composite of the Virgo Cluster about 65 million light-years away.

The telescope’s six color filters will reveal stellar streams, remnants of dwarf galaxies and clusters the Milky Way has consumed, while gaps and density variations in those streams probe dark-matter clumps, and Rubin will combine its data with Gaia space telescope measurements to create a six-dimensional map showing where stars are and how they are moving. By measuring precise positions, motions, colors and brightness variations of billions of stars, astronomers will reconstruct how the Milky Way formed and evolved over billions of years.

The researchers said early Rubin Observatory data could reveal new understanding of how faint galaxies change across their lifetimes. The Aquarius IV and Rubin-GC1 studies drew on researchers from the University of Chicago, the University of Virginia, the University of Washington, Dartmouth College, the University of Tampa, NSF NOIRLab, Duke University, the University of Toronto, the Instituto de Astrofísica de Andalucía in Spain, the University of Colorado-Boulder and Lund University in Sweden. The collaboration spanned institutions in the United States, Spain and Sweden.