Two-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 recently discovered counterrotating spiral and SO galaxies. Strong linear two-stream instability of tightly wrapped spiral waves is found for one- and two-armed waves with the pattern angular speed of the unstable waves always intermediate between the angular speed of the corotating matter (+Omega) and that of the counterrotating matter (-Omega). The instability arises from the interaction of positive and negative energy modes 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 important than the local growth rate. For a galaxy of corotating stars and counterrotating stars of mass fraction xi(g), < 1/2, or of counterrotating gas of mass fraction xi(g), < 1/2, the largest amplification is usually for the one-armed leading waves (with respect to the corotating stars). For the case of both counterrotating stars and gas, the largest amplifications are for xi* + xi(g) approximate to 1/2, also for one-armed leading waves. The two-armed trailing waves usually have smaller 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.