The Vera Rubin Has Found Two More of the Milky Way's Dim Satellites

The Vera Rubin Observatory has detected two more dim satellites of the Milky Way: Aquarius IV, a likely dwarf galaxy, and Rubin-GC1, a globular cluster. These discoveries are part of the Early Data Preview 2 (EDP2) released in July 2026. Aquarius IV is a low-luminosity, compact stellar system in the outer Galactic halo, with an ancient and low-metallicity stellar population. Rubin-GC1 is in the direction of the Milky Way's bulge, with a dense stellar population and thick dust. The detections are presented in two new papers, one about Aquarius IV and one about Rubin-GC1.

• "Discovery of the Distant, Ultra-faint Milky Way Satellite Aquarius IV with the Vera C. Rubin Observatory Early Data Preview 2." It's published in Research Notes of the American Astronomical Society, and the lead author is William Cerny, an astronomy student in Yale’s Graduate School of Arts and Sciences. "We present the discovery of Aquarius IV (Rubin J2201−0234)—the first ultra-faint Milky Way (MW) satellite to be identified using data from the Vera C. Rubin Observatory," the authors write. It was confirmed in images from the Dark Energy Camera. The authors say it's a low-luminosity, compact stellar system in the outer Galactic halo. It's stellar population is ancient and low-metallicity. "These properties closely resemble those of the smallest and faintest confirmed ultra-faint dwarf galaxies (UFDs), though a globular cluster classification is not ruled out," the researchers write.

• "Rubin Observatory Reveals a Dust-Shrouded Halo Globular Cluster in Ophiuchus," and it will be published in The Astrophysical Journal Letters. It's currently available at arxiv.org and the lead author is Aashay Pai. Pai is a graduate student in the Department of Physics at the University of Chicago. "We present the discovery of the Milky Way satellite Rubin-GC1 (Rubin J1628−1735) using data from the NSF-DOE Vera C. Rubin Observatory’s Early Data Preview 2," the authors write. The stars in Rubin-GC1 have an intermediate age of about 9.2 billion years, and are moderately rich in metals, meaning it's most likely a very faint GC. "Together, these properties suggest Rubin-GC1 is most likely a star cluster in the faint tail of the luminosity function of globular clusters surrounding the Milky Way," the authors write. Based on comparisons to the Sagittarius dwarf spheroidal galaxy, Rubin-GC1 is most likely a cluster accreted to the Milky Way, associated with the Sagittarius leading arm. The Sagittarius leading arm is a massive tidal stream of stars from the Sagittarius Dwarf Spheroidal Galaxy, torn away as that galaxy merges with the Milky Way.

These discoveries are significant as they provide new insights into the assembly history of the Milky Way through mergers. The Vera Rubin Observatory's EDP2 data has enabled the detection of these satellites, which are crucial for understanding the formation and evolution of galaxies. The findings highlight the potential of the observatory to uncover more satellites in the future, contributing to a more complete census of satellite galaxies around the Milky Way. The detection of Rubin-GC1 in the direction of the galactic bulge, where stellar density is high and dust is thick, makes the discovery even more impressive. These findings support the idea that dark matter plays a crucial role in the formation and evolution of galaxies through mergers.

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