Yale study of Rubin Observatory data finds two hidden companions of the Milky Way

Yale researchers have identified two previously unknown companions of the Milky Way using preview data from the recently completed NSF-DOE Vera C. Rubin Observatory in Chile. The findings are an early sign that the observatory could help complete the census of the galaxy’s faintest neighbors.

Two new studies, including one led by a Yale astronomer, describe the discoveries. One is a galaxy called Aquarius IV, located in the Milky Way’s distant outskirts, roughly 355,000 light-years from Earth. The other, named Rubin-GC1, is a star cluster on the far side of the Milky Way, inside a region shrouded by cosmic dust.

Large galaxies are often orbited by satellites, which are small galaxies or star clusters, but many are extremely faint and hard to find. About 65 satellite galaxies of the Milky Way are known, and there is speculation that hundreds more remain undetected.

William Cerny, an astronomy student in Yale’s Graduate School of Arts and Sciences, is 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. “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.

Marla Geha, a professor of astronomy and physics in Yale’s Faculty of Arts and Sciences, is a co-author on 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,” Geha said. Aashay Pai of the University of Chicago is the first author of the Rubin-GC1 study.

Astronomers analyzed Rubin preview data taken between April 2025 and January 2026, which covered about 7% of the total night sky. The team included researchers from the University of Chicago, the University of Virginia, the University of Washington, Dartmouth College, the University of Tampa, the 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 researchers say the findings may also shed light on 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 of a larger galaxy, are almost entirely composed of dark matter.

The Rubin Observatory began its 10-year Legacy Survey of Space and Time in June. One of its main goals is to map the Milky Way’s halo, the cloudlike sphere of gas, stars and dark matter surrounding the main part of the galaxy, and to find hidden satellite galaxies, star clusters and other substructures.

The observatory sits on Cerro Pachón in the Chilean Andes and has an 8.4-meter Simonyi Survey Telescope with the largest digital camera ever built, at 3.2 gigapixels. It is imaging the Southern Hemisphere sky to create an ultra-wide, time-lapse record of the night sky. The National Science Foundation and the U.S. Department of Energy’s Office of Science jointly fund it.

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Yale study of Rubin Observatory data finds two hidden companions of the Milky Way