Instability of counter-rotating stellar disks
  We use an N-body simulation, constructed using GADGET-2, to investigate an
accretion flow onto an astrophysical disk that is in the opposite sense to the
disk's rotation. In order to separate dynamics intrinsic to the
counter-rotating flow from the impact of the flow onto the disk, we consider an
initial condition in which the counter-rotating flow is in an annular region
immediately exterior the main portion of the astrophysical disk. Such
counter-rotating flows are seen in systems such as NGC 4826 (known as the "Evil
Eye Galaxy"). Interaction between the rotating and counter-rotating components
is due to two-stream instability in the boundary region. A multi-armed spiral
density wave is excited in the astrophysical disk and a density distribution
with high azimuthal mode number is excited in the counter-rotating flow.
Density fluctuations in the counter-rotating flow aggregate into larger clumps
and some of the material in the counter-rotating flow is scattered to large
radii. Accretion flow processes such as this are increasingly seen to be of
importance in the evolution of multi-component galactic disks.
