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

## TL;DR

This paper characterizes FSR 1758, a recently discovered massive object in the Galactic Bulge, exploring whether it is a large globular cluster or a dwarf galaxy, with implications for understanding galactic structure.

## Contribution

It provides detailed measurements and analysis of FSR 1758's properties, proposing its potential classification as either a giant globular cluster or a dwarf galaxy, and discusses its significance.

## Key findings

- FSR 1758 is a large, metal-poor globular cluster candidate.
- It exhibits properties similar to omega Centauri, including size and stellar populations.
- Possible association with tidal debris or a larger dwarf galaxy structure.

## Abstract

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.