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 .