# Rubin Observatory test data reveal Aquarius IV, a faint Milky Way satellite candidate, at 8 sigma - Published: 2026-11-13 22:46 UTC - Section: Sky & Space › Astronomy - Tags: Aquarius IV, Rubin Observatory test data, Milky Way satellite, ultra-faint dwarf galaxy, Early Data Preview 2, Dark Energy Camera, globular cluster, William Cerny, Yale astronomy - URL: https://realtech.media/news/rt-80b3bb684752813f90e74f5e_rubin_observatory_test/ **A Yale-led team found Aquarius IV in Vera C. Rubin Observatory test data; the low-luminosity stellar system may be an ultra-faint dwarf galaxy or a globular cluster.** ![Rubin Observatory test data reveal Aquarius IV, a faint Milky Way satellite candidate, at 8 sigma](https://realtech.media/media/2026/11/9897736a43359150.jpg) *Whyistheskyblue1, Wikimedia Commons, CC BY-SA 4.0* A team led by Yale graduate student William Cerny reported Aquarius IV (Rubin J2201−0234), the first ultra-faint Milky Way satellite identified in data from the Vera C. Rubin Observatory, in Research Notes of the AAS [1]. The system was detected at about 8 sigma in Rubin Early Data Preview 2 photometry and independently confirmed at about 6 sigma in archival Dark Energy Camera imaging [1]. The data behind the find covered about 7 percent of the night sky. The observatory on Cerro Pachón in Chile released the preview on July 27, 2026. Seven days later Cerny and 18 co-authors posted the paper. The release covers about 3,000 square degrees, imaged between April 2025 and January 2026, before the 10-year Legacy Survey of Space and Time started in June 2026. ## How the object was found The team selected blue stars in parts of the sky with little dust. They then ran a search called simple, which looks for small clumps of stars with the colors of a very old population. The authors assumed an age of 13 billion years and scanned distances in steps. The search was repeated with two pairs of color filters, and the lists were compared to weed out false alarms. Aquarius IV ranked highest among the new detections, at 8.0 sigma in one filter pair and 7.6 sigma in the other. The same run recovered all three known ultra-faint satellites in that area: Aquarius II, Aquarius III and Virgo III. After the spot was flagged, stacked Rubin images and archival Dark Energy Camera images both showed an excess of faint blue stars at the same position. The object was also on a list of candidates from an earlier search of Dark Energy Camera data, at about 6 sigma. ## Properties and open questions The fit gives Aquarius IV an absolute magnitude of about −1.9 and a half-light radius of about 19 parsecs [1]. Its stars are consistent with an ancient, metal-poor population of 13 billion years [1]. The distance of about 355,000 light-years rests on a single likely blue horizontal branch star, located 0.8 arcminutes from the center of the object. A footnote in the paper covers the other case: if that star is not a member, distances of about 490,000 to 650,000 light-years may fit better. The properties resemble those of the smallest and faintest confirmed ultra-faint dwarf galaxies, though a globular cluster classification is not ruled out [1]. The radius exceeds that of nearly all Milky Way globular clusters. The authors call their measurements preliminary and say deeper imaging and spectroscopy are needed to settle the properties and classification [1]. The note appeared in a journal that the American Astronomical Society describes as moderated and not peer-reviewed. Yale counts about 65 known satellite galaxies of the Milky Way, and some researchers suspect hundreds more remain undetected. The paper expects the full survey to catch systems like Aquarius IV with better than 85 percent efficiency. Rubin's main survey has more than nine years left to run. ## Sources 1. [Research Notes of the AAS](https://doi.org/10.3847/2515-5172/ae993b) 2. [Earth.com](https://www.earth.com/space/aquarius-iv-faint-galaxy-rubin-observatory) --- Source: RealTech, realtech.media — https://realtech.media/news/rt-80b3bb684752813f90e74f5e_rubin_observatory_test/