A Gravitational Double Scattering Mechanism for Generating High Velocity
  Objects
  We present a dynamical model describing how halo particles can receive a
significant energy kick from the merger between their own host halo and a
target halo. This is highly relevant for understanding the growth of
cosmological halos, and could especially provide an explanation for some high
velocity objects. The model we present includes a \emph{double scattering
mechanism}, where a halo particle is given a significant energy kick by
undergoing two subsequent gravitational deflections during the merger. The
first deflection is by the potential of the target halo, whereas the second is
by the potential of the particle's original host halo. The resultant energy
kick arises because the two halos move relative to each other during the two
deflections. To our knowledge, this mechanism has never been characterized in
this context before. We derive analytically a halo particle's total kick
energy, which is composed of energy from the double scattering mechanism and
energy release from tidal fields, as a function of its position in its original
host halo. In the case of a $1:10$ merger between two Hernquist halos, we
estimate that the presented mechanisms can generate particles with a velocity
$\sim 2$ times the virial velocity of the target halo measured at its virial
sphere. This motivates us to suggest that the high velocity of the recently
discovered globular cluster HVCG-1 \citep{2014ApJ...787L..11C} can be explained
by a head-on halo merger. Finally, we illustrate the orbital evolution of
particles outside the virial sphere of the target halo, by solving the equation
of motion in an expanding universe. We find a 'sweet spot' around a scale
factor of $0.3-0.5$ for ejecting particles into large orbits, which easily can
reach beyond $\sim 5$ virial radii.
