新型天文台が天の川銀河に隠れていた2つの伴銀河を発見（Historic new observatory reveals two hidden companions of the Milky Way）

2026-10-28 イェール大学

米国イェール大学などの国際研究チームは、チリに建設されたNSF・DOEベラ・C・ルービン天文台の試験観測データから、天の川銀河に属する未発見の伴天体2個を発見した。1つは地球から約35万5,000光年離れた超微光矮小銀河「Aquarius IV」、もう1つは銀河系の反対側に位置し、宇宙塵に隠された星団「Rubin-GC1」である。研究チームは2025年4月から2026年1月までに取得された、全天の約7％に相当する観測データを解析し、自動検出アルゴリズムによって極めて暗い天体を特定した。ルービン天文台は口径8.4 mの望遠鏡と32億画素の大型デジタルカメラを備え、今後10年間にわたり南天を繰り返し観測する。今回の発見は、これまで見逃されてきた多数の伴銀河や星団の検出可能性を示すものであり、銀河形成の過程、天の川銀河の進化史、暗黒物質の性質の解明に貢献すると期待される。

A skymap of stars in the Milky Way and its environment, using data from the Gaia satellite. The area of sky observed by Rubin as part of “Data Preview 2” is shaded in red. The positions of Rubin-GC1 and Aquarius IV appear in orange and blue, respectively. Rubin-GC1 stands out prominently as a collection of numerous resolved stars. Aquarius IV, which is much more distant, is only barely detectable by eye in the image, but was identified at high confidence by automated search algorithms. Credit: NSF-DOE Vera C. Rubin Observatory/NOIRLab/SLAC/AURA, ESA/Gaia/DPAC, W. Cerny

＜関連情報＞

ベラ・C・ルービン天文台による遠方の超微弱な天の川衛星アクエリアスIVの発見　早期データプレビュー2 Discovery of the Distant, Ultra-faint Milky Way Satellite Aquarius IV with the Vera C. Rubin Observatory Early Data Preview 2

William Cerny, Aashay Pai, Alex Drlica-Wagner, Andrew B. Pace, Peter S. Ferguson, Marla Geha, Chin Yi Tan, Sasha Campana, Jeffrey L. Carlin, Denija Crnojević,…
Research Notes of the AAS Published: August 2026
DOI:10.3847/2515-5172/ae993b

Abstract

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. This system was detected at ∼8σ significance using Rubin Early Data Preview 2 (EDP2) photometry and independently confirmed at ∼6σ significance in archival Dark Energy Camera imaging. Jointly fitting its morphology and distance, we find that Aquarius IV is a low-luminosity (Mv = 1.9^+0.6_-1.0), compact (r_1/2=19⁺⁴_-6 pc; r_{h =}0.60^+0.14_-0.17 arcmin) stellar system in the outer Galactic halo (D_⊙ = 109⁺⁶_-8kpc). Its stellar population is consistent with an ancient, metal-poor stellar isochrone (τ = 13 Gyr, Z = 0.0001). 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. Given the small number of detected member stars in Rubin EDP2, deeper imaging and spectroscopy will be critical for determining the properties and classification of Aquarius IV at higher confidence.