  Motivated by observations of disk galaxies with counter-rotating stars, we
have run two-dimensional, collisionless N-body simulations of disk galaxies
with significant counter-rotating components. For all our simulations the
initial value of Toomre's stability parameter was Q = 1.1. The percentage of
counter-rotating particles ranges from 25% to 50%. A stationary one-arm spiral
wave is observed to form in each run, persisting from a few to five rotation
periods, measured at the half-mass radius. In one run, the spiral wave was
initially a leading arm which subsequently transformed into a trailing arm. We
also observed a change in spiral direction in the run initially containing
equal numbers of particles orbiting in both directions. The results of our
simulations support an interpretation of the one armed waves as due to the two
stream instability.

Astrophysics