ONE-ARMED SPIRAL WAVES IN GALAXY SIMULATIONS WITH COUNTERROTATING STARS

Motivated by observations of disk galaxies with counterrotating stars, we have run two-dimensional, collisionless N-body simulations of disk galaxies with significant counterrotating components. For all our sim ulations the initial value of Toomre's stability parameter was Q = 1.1 . The percentage of counterrotating particles ranges from 25% to 50%. A stationary one-arm spiral wave is observed to form in each run, pers isting from a few to five rotation periods, measured at the half-mass radius. In one run, the spiral wave was initially a leading arm that s ubsequently 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 being due to the t wo-stream instability.