Astronomers have identified two satellites of the Milky Way that had gone unnoticed until now: a faint, sparse dwarf galaxy named Aquarius IV and a globular cluster of stars wrapped in dust. Both discoveries were made with data from the Vera Rubin Observatory in Chile, whose 8.4-meter telescope began operating last year. The findings are reported in two new studies, one published in Research Notes of the AAS and the other in The Astrophysical Journal Letters.
William Cerny, an astronomy student at the Graduate School of Arts and Sciences of Yale University in Connecticut, led the work on Aquarius IV. The globular cluster, which the team named Rubin-GC1, was identified in a study led by Aashay Pai of the University of Chicago. The research teams analyzed observations collected between April 2025 and January of this year, covering about 7 percent of the night sky.
Aquarius IV lies more than 350,000 light-years from Earth, in the distant outskirts of the Milky Way. It is an ultra-faint galaxy, and the team needed the observatory's sensitivity to detect it at all. Rubin-GC1, by contrast, is a globular cluster, a roughly spherical grouping of stars, and sits in the region of the galaxy farthest from the Sun. It is the first such cluster discovered by the observatory.
Cerny said the preliminary data already show the observatory can pick out systems this faint and difficult to detect, and that these discoveries are only a beginning. The observatory's full survey, the Legacy Survey of Space and Time, will photograph the entire southern sky repeatedly over ten years using its 3,200-megapixel camera, the largest ever built. The program began in June and will produce a sort of vast film of the southern sky showing how it changes over the years. Cerny expects it to reveal many more satellites of the Milky Way.
One of the survey's scientific goals is to map the halo of the Milky Way, a huge structure of gas, stars and dark matter in which the galaxy is embedded. This mapping is expected to reveal substructures, groups of stars and small galaxies not yet catalogued. Some theories suggest there could be hundreds of satellites orbiting galaxies like ours, so the two objects may be the first of a long series.
Marla Geha, co-author of both papers, said that finding new Milky Way satellites in Rubin data had long been an exciting prospect for much of her career, and that it is remarkable it has now become reality. The discoveries could help scientists learn more about dark matter, the invisible component that makes up more than a quarter of the universe. Faint, diffuse galaxies such as Aquarius IV are thought to be rich in this elusive substance. Geha said further identifications will allow astronomers to complete the census of satellite galaxies around the Milky Way, a key step toward understanding how the faintest galaxies form.
Source: media.inaf.it
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