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) about (359, 21) in a region of high stellar density and a foreground reddening of E(B-V) about 0.7. We simultaneously fit the morphology and isochrone parameters of Rubin-GC1, determining that it is a faint (M V = -2.4+0.5-0.7) and compact (r 1/2 = 5+1-1 pc) Milky Way satellite at a distance of D = 31+3-3 kpc; its stellar population is intermediate-age (τ=9.2+1.2-0.7 Gyr) and moderately metal-rich ([Fe/H] iso=-0.73+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.