2-STREAM INSTABILITY OF COUNTERROTATING GALAXIES

The present study of the two-stream instability in stellar disks with counterrotating components of stars and/or gas is stimulated by recent ly discovered counterrotating spiral and SO galaxies. Strong linear tw o-stream instability of tightly wrapped spiral waves is found for one- and two-armed waves with the pattern angular speed of the unstable wa ves always intermediate between the angular speed of the corotating ma tter (+Omega) and that of the counterrotating matter (-Omega). The ins tability arises from the interaction of positive and negative energy m odes in the co- and counterrotating components. The unstable waves are in general convective-they move in radius and radial wavenumber space -with the result that amplification of the advected wave is more impor tant than the local growth rate. For a galaxy of corotating stars and counterrotating stars of mass fraction xi(g), < 1/2, or of counterrota ting gas of mass fraction xi(g), < 1/2, the largest amplification is u sually for the one-armed leading waves (with respect to the corotating stars). For the case of both counterrotating stars and gas, the large st amplifications are for xi + xi(g) approximate to 1/2, also for one -armed leading waves. The two-armed trailing waves usually have smalle r amplifications. The growth rates and amplifications all decrease as the velocity spreads of the stars and/or gas increase. It is suggested that the spiral waves can provide an effective viscosity for the gas causing its accretion.