A Sequoia in the Garden: FSR 1758—Dwarf Galaxy or Giant Globular Cluster?

We present the physical characterization of FSR 1758, a new large, massive object very recently discovered in the Galactic Bulge. The combination of optical data from the 2nd Gaia Data Release and the DECam Plane Survey and near-IR data from the VISTA Variables in the Vía Láctea Extended Survey led to a clean sample of likely members. Based on this integrated data set, position, distance, reddening, size, metallicity, absolute magnitude, and proper motion (PM) of this object are measured. We estimate the following parameters: α = 17:31:12, δ = −39:48:30 (J2000), D = 11.5 ± 1.0 kpc, $E(J-{Ks})=0.20\pm 0.03$ mag, Rc = 10 pc, Rt = 150 pc, $[\mathrm{Fe}/{\rm{H}}]=-1.5\pm 0.3$ dex, Mi < −8.6 ± 1.0, μα = −2.85 mas yr−1, and μδ = 2.55 mas yr−1. The nature of this object is discussed. If FRS 1758 is a genuine globular cluster (GC), it is one of the largest in the Milky Way, with a size comparable or even larger than that of ω Cen, being also an extreme outlier in the size versus Galactocentric distance diagram. The presence of a concentration of long-period RR Lyrae variable stars and blue horizontal branch stars suggests that it is a typical metal-poor GC of Oosterhoff type II. Further exploration of a larger surrounding field reveals common PM stars, suggesting either tidal debris or that FRS 1758 is actually the central part of a larger extended structure such as a new dwarf galaxy, tentatively named Scorpius. In either case, this object is remarkable, and its discovery graphically illustrates the possibility of finding other large objects hidden in the Galactic Bulge using future surveys.