Preview data from the Simonyi Survey Telescope at the NSF-DOE Vera C. Rubin Observatory point to a wave of discoveries that may complete the census of the Milky Way's neighboring galaxies - including its faintest companions.
Using early data from Rubin, a pair of recent studies have identified two ultra-faint and previously unknown satellites of the Milky Way. One study, published recently in Research Notes of the AAS, identifies a galaxy called Aquarius IV, located in the Milky Way's distant outskirts. The second study, published Oct. 28 in The Astrophysical Journal Letters, identifies Rubin-GC1, a star cluster located on the far side of the Milky Way, inside a murky region shrouded by cosmic dust.
Large galaxies are often orbited by satellites - small galaxies or star clusters - but many are extremely faint and difficult to find. The Simonyi Survey Telescope's 3.2 gigapixel sensor - the largest digital camera ever built - can bring many more of those faint satellites in view. More than 20 researchers at the University of Washington have worked to get in-focus images from the telescope, validate data ahead of releases and maintain an accurate catalog of known stars, galaxies and other phenomena.
"Finding something as faint as Aquarius IV comes down to how clean your stellar catalog is," said Peter Ferguson, a UW postdoctoral scholar in astronomy and co-author of both studies. "A handful of misclassified galaxies or spurious detections can wash out the signal entirely, so much of our work at the UW was about making sure that when Rubin tells you a point of light is a star, you can believe it."
The Rubin Observatory began its 10-year Legacy Survey of Space and Time (LSST) in June. One of its main missions is to map what is known as the Milky Way's "halo" - the cloudlike sphere of gas, stars and dark matter that surrounds the main part of the galaxy - and find previously hidden satellite galaxies, star clusters and other substructures. There are about 65 known satellites of the Milky Way, yet there is speculation that hundreds more Milky Way satellites remain undetected.
"What I find especially exciting is that these discoveries are just the beginning for Rubin," said William Cerny, a graduate student of astronomy at Yale University and lead author of the Aquarius IV study. "These new satellites will give us new opportunities to test models of galaxy formation, trace the assembly history of the Milky Way and investigate the nature of dark matter."
For the two new studies, astronomers analyzed Rubin Observatory preview data taken between April 2025 and January 2026.
"Excitingly, these two objects came out of test data covering just 15% of the full LSST footprint," Ferguson said. "Now that the LSST is underway, we'll be able to push to fainter and more distant systems than any survey before. Aquarius IV and Rubin-GC1 are a preview of what the next decade will look like."
Beyond the significance of the discoveries themselves, the researchers say their findings may provide valuable information about the nature of dark matter, the invisible scaffolding that makes up most of the matter in the universe. Ultra-faint galaxies such as Aquarius IV, which have a fraction of the stars that a larger galaxy contains, are almost entirely composed of dark matter.
Likewise, the researchers say, Rubin's early data may yield new insight into the life cycle of some of the universe's hardest-to-spot galaxies.
"These discoveries are incredibly promising," said Marla Geha, a professor of astronomy and physics at Yale University who co-authored both studies. "Continued discoveries will allow us to complete the census of satellite galaxies around the Milky Way, which is a key to understanding how the faintest galaxies form."
UW co-authors include Nora Shipp, an assistant professor of astronomy and Chin-Yi Tan, a postdoctoral scholar in astronomy. A complete list of co-authors is included with the studies.
Operations of the Vera C. Rubin Observatory are funded by the U.S. National Science Foundation and the U.S. Department of Energy's Office of Science.
