Rubin Test Data Reveals Two Faint Milky Way Companions
Astronomers using preview images from the Rubin Observatory found a faint satellite galaxy and a dust-hidden star cluster around the Milky Way.
Two studies based on preview data from the Vera C. Rubin Observatory in Chile report two previously unknown companions of the Milky Way. The researchers see it as an early sign that the observatory could uncover the hundreds of faint companions thought to be hiding around the galaxy.
Large galaxies are often orbited by satellites, meaning small galaxies or star clusters, but many are extremely faint. About 65 satellite galaxies of the Milky Way are known. There is speculation that hundreds more remain undetected.
Two new objects
One find is a galaxy named Aquarius IV, in the Milky Way's distant outskirts about 355,000 light-years from Earth. The other, Rubin-GC1, is a star cluster on the far side of the Milky Way, inside a region shrouded by cosmic dust.
The team analyzed preview images taken between April 2025 and January 2026. They covered about 7% of the night sky.
William Cerny, a Yale graduate student, is first author of the Aquarius IV paper in Research Notes of the AAS and second author of the Rubin-GC1 study. That study, led by Aashay Pai of the University of Chicago, has been accepted by The Astrophysical Journal Letters and is available as an arXiv preprint.
"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.
Why it matters to the researchers
Rubin began its 10-year Legacy Survey of Space and Time in June. Mapping the Milky Way's halo and finding hidden satellites is among its main aims. The observatory has an 8.4-meter telescope and a 3.2-gigapixel camera.
Marla Geha, a Yale professor and co-author on both studies, called the results "incredibly promising". She said continued discoveries will allow astronomers to "complete the census of satellite galaxies around the Milky Way". Ultra-faint galaxies such as Aquarius IV are almost entirely made of dark matter, so they may also say something about its nature.
Source: Research Notes of the AAS