# The retrograde orbit of the globular cluster FSR1758 revealed with Gaia DR2

## TL;DR

This study reveals that the globular cluster FSR1758 has a retrograde, radial orbit around the Milky Way, based on Gaia DR2 data, indicating it is a halo intruder rather than a dwarf galaxy.

## Contribution

First radial velocity measurements of FSR1758 establish its orbit and nature, confirming it as a globular cluster on a retrograde trajectory.

## Key findings

- FSR1758 has a radial velocity of 227 km/s.
- It follows a very retrograde, radial orbit with specific pericentre and apocentre.
- Most stars in the surrounding 'halo' are foreground dwarfs.

## Abstract

We report the first radial velocity measurements of the recently identified globular cluster FSR1758. From the two member stars with radial velocities from the Gaia Radial Velocity Spectrograph reported in Gaia DR2, we find FSR1758 has a radial velocity of $227\pm1$ km/s. We also find potential extra-tidal star lost from the cluster in the surrounding 1~deg. Combined with Gaia proper motions and photometric distance estimates, this shows that FSR1758 is on a very retrograde, radial orbit with an pericentre of $3.8_{-0.9}^{+0.9}$ kpc, an apocentre of $16_{-5}^{+8}$ kpc, and eccentricity of $0.62_{-0.04}^{+0.05}$. Although it is currently at a Galactocentric distance of $3.8_{-0.9}^{+0.9}$ kpc --- at the edge of the bulge --- it is an intruder from the halo. We investigate whether a reported `halo' of stars around FSR1758 is related to the cluster, and find that most of these stars are likely foreground dwarf stars. We conclude that FSR1758 is not a dwarf galaxy, but rather a globular cluster.