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

Abstract

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 $(l, b) \sim (359^{\circ}, 21^{\circ})$ in a region of high stellar density and a foreground reddening of $E(B-V) \approx 0.7$. 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 $D_\odot = 31^{+3}_{-3}$ kpc; its stellar population is intermediate-age ($τ=9.2^{+1.2}_{-0.7}$ Gyr) and moderately metal-rich ($\mathrm{[Fe/H]_{iso}}=-0.73^{+0.04}_{-0.04}$). 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.