Could the Hercules satellite be a stellar stream in the Milky Way halo?

TL;DR

This paper explores whether the Hercules satellite is a stellar stream formed by tidal disruption, using velocity data and orbital analysis to support the hypothesis of its recent transformation in the Milky Way halo.

Contribution

It provides the first detailed orbital analysis suggesting Hercules is a stellar stream resulting from tidal disruption in the Milky Way.

Findings

Hercules likely experienced a close approach to the Galactic center about 0.6 Gyr ago.

The orbit indicates strong tidal forces could have transformed Hercules into a stellar stream.

Velocity data constrains the tangential velocity and supports the tidal disruption scenario.

Abstract

We investigate the possibility that Hercules, a recently discovered Milky Way (MW) satellite, is a stellar stream in the process of formation. This hypothesis is motivated by Hercules' highly elongated shape as well as the measurement of a tentative radial velocity gradient along its body. The application of simple analytical techniques on radial velocity data of its member stars provides tight constraints on the tangential velocity of the system (v_t = -16^{+6}_{-22} km/s, relative to the Galactic Standard of Rest). Combining this with its large receding velocity (145 km/s) and distance (138 kpc) yields an orbit that would have taken Hercules to within 6^{+9}_{-2} kpc of the Galactic centre approximately 0.6 Gyr ago. This very small perigalacticon can naturally explain the violent tidal destruction of the dwarf galaxy in the MW's gravitational potential, inducing its transformation into a stellar stream.