Rubin Observatory Reveals a Dust-shrouded Halo Globular Cluster in Ophiuchus

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 system was discovered in the direction of the Galactic bulge with ( l , b ) ∼ (359°, 21°) in a region of high stellar density and a foreground reddening of E ( B − V ) ≈ 0.7. We simultaneously fit the morphology and isochrone parameters of Rubin-GC1, determining that it is a faint ( M V = − 2 . 4 − 0.7 + 0.5 ) and compact ( r 1 / 2 = 5 − 1 + 1 pc) Milky Way satellite at a distance of D ⊙ = 3 1 − 3 + 3 kpc; its stellar population is intermediate-age ( τ = 9 . 2 − 0.7 + 1.2 Gyr) and moderately metal-rich ( [ Fe / H ] iso = − 0.7 3 − 0.04 + 0.04 ). 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. We identify a distinct proper motion signature from Rubin-GC1 in Gaia DR3, and from a comparison of the system’s 5D phase space information to N -body models of the Sagittarius dwarf spheroidal galaxy’s tidal stream, we determine that Rubin-GC1 is likely an accreted cluster associated with the Sagittarius leading arm. The discovery of Rubin-GC1 highlights the potential of Rubin to uncover faint, resolved satellites in dust-shrouded regions of the Milky Way, including remnants of the accretion events that have shaped the Galaxy’s hierarchical assembly.