Rubin observatory reveals two hidden companions of the Milky Way

A sky map of stars in the Milky Way and its environment, using data from the Gaia satellite. The area of sky observed by Rubin as part of "Data Preview 2" is shaded in red. The positions of Rubin-GC1 and Aquarius IV appear in orange and blue, respectively. Rubin-GC1 stands out prominently as a collection of numerous resolved stars. Aquarius IV, which is much more distant, is only barely detectable by eye in the image but was identified at high confidence by automated search algorithms. Credit: NSF-DOE Vera C. Rubin Observatory/NOIRLab/SLAC/AURA, ESA/Gaia/DPAC, W. Cerny Preview data from the recently completed NSF-DOE Vera C. R. Observatory in Chile point to a wave of discoveries that may complete the census of the Milky Way's neighboring galaxies—including its faintest companions. Using newly available data from the observatory, a ground-based astrophysics facility on the mountain of Cerro Pachón in the Chilean Andes, researchers identified two previously unknown companions of the Milky Way, an early indicator that the observatory's advanced technology could reveal the hundreds of faint companions thought to be hiding around Earth's home galaxy. Large galaxies are often orbited by satellites—small galaxies or star clusters—but many are extremely faint and difficult to find. There are about 65 known satellite galaxies of the Milky Way, yet there is speculation that hundreds more Milky Way satellites remain undetected. A pair of new studies—including one led by a Yale astronomer—suggests that the Rubin Observatory may be able to provide more clarity. Two faint companions emerge from preview data Using test data taken prior to Rubin's official launch, the studies have identified two ultra-faint Milky Way satellites. One is a galaxy called Aquarius IV, located in the Milky Way's distant outskirts, roughly 355,000 light-years from Earth. The other satellite, named Rubin-GC1, is a star cluster located on the far side of the Milky Way, inside a murky region shrouded by cosmic dust. "What I find especially exciting is that these discoveries are just the beginning for Rubin," said William Cerny, an astronomy student in Yale's Graduate School of Arts and Sciences and first author of the Aquarius IV study in the journal Research Notes of the AAS. He is second author of the Rubin-GC1 study, which has been accepted by The Astrophysical Journal Letters and is currently available in preprint on arXiv. "If the observatory can find systems this faint and difficult with only its preview data, its full 10-year survey will undoubtedly provide us with many more Milky Way satellite discoveries," Cerny said. "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." A decade mapping the Milky Way's halo 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 Milky Way—and find previously hidden satellite galaxies, star clusters, and other substructures. The observatory is equipped with an 8.4-meter Simonyi Survey Telescope, featuring the largest digital camera ever built, with 3.2 gigapixels. It is taking detailed images of the Southern Hemisphere sky with the goal of creating an ultra-wide, time-lapse record of the night sky. For the two new studies, astronomers analyzed Rubin Observatory preview data taken between April 2025 and January 2026. The data covered about 7% of the total night sky. "Finding new Milky Way satellites in Rubin Observatory data has been an exciting future prospect for most of my career," said Marla Geha, a professor of astronomy and physics in Yale's Faculty of Arts and Sciences and co-author on both studies. "It is remarkable this is now a reality." The team included researchers from the University of Chicago, the U

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