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 (GDR2) and the DECam Plane Survey
(DECaPS), and near-IR data from the VISTA Variables in the V\'{\i}a L\'actea
Extended Survey (VVVX) led to a clean sample of likely members. Based on this
integrated dataset, position, distance, reddening, size, metallicity, absolute
magnitude, and proper motion of this object are measured. We estimate the
following parameters: $\alpha=17:31:12$, $\delta=-39:48:30$ (J2000), $D=11.5
\pm 1.0$ kpc, $E(J-Ks)=0.20 \pm 0.03$ mag, $R_c=10$ pc, $R_t=150$ pc,
$[Fe/H]=-1.5 \pm 0.3$ dex, $M_i < -8.6 \pm 1.0$, $\mu_{\alpha} = -2.85$ mas
yr$^{-1}$, and $\mu_{\delta} = 2.55$ mas yr$^{-1}$. The nature of this object
is discussed. If FRS 1758 is a genuine globular cluster, it is one of the
largest in the Milky Way, with a size comparable or even larger than that of
$\omega$ Cen, being also an extreme outlier in the size vs. 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 globular cluster of Oosterhoff type II. Further exploration of a
larger surrounding field reveals common proper motion 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 as Scorpius.
In either case, this object is remarkable, and its discovery graphically
illustrates the possibility to find other large objects hidden in the Galactic
Bulge using future surveys.
