Exploding Satellites -- The Tidal Debris of the Ultra-Faint Dwarf Galaxy
  Hercules
  The ultra-faint satellite galaxy Hercules has a strongly elongated and
irregular morphology with detections of tidal features up to 1.3 deg (3 kpc)
from its center. This suggests that Hercules may be dissolving under the Milky
Way's gravitational influence, and hence could be a tidal stream in formation
rather than a bound, dark-matter dominated satellite. Using Bayesian inference
in combination with N-body simulations, we show that Hercules has to be on a
very eccentric orbit (epsilon~0.95) within the Milky Way in this scenario. On
such an orbit, Hercules "explodes" as a consequence of the last tidal shock at
pericenter 0.5 Gyr ago. It is currently decelerating towards apocenter of its
orbit with a velocity of V=157 km/s -- of which 99% is directed radially
outwards. Due to differential orbital precession caused by the non-spherical
nature of the Galactic potential, its debris fans out nearly perpendicular to
its orbit. This explains why Hercules has an elongated shape without showing a
distance gradient along its main body: it is in fact a stream that is
significantly broader than it is long. In other words, it is moving
perpendicular to its apparent major axis. In this scenario, there is a spike in
the radial velocity profile created by the dominant debris component that
formed through the last pericenter passage. This is similar to kinematic
substructure that is observed in the real Hercules. Modeling a satellite on
such a highly eccentric orbit is strongly dependent on the form of the Galactic
potential. We therefore propose that detailed kinematic investigation of
Hercules and other exploding satellite candidates can yield strong constraints
on the potential of the Milky Way.
