Two‐Stream Instability of Counterrotating Galaxies

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Abstract
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 S0 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 (+Ω) and that of the counterrotating matter (-Ω). 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 ξ*<½, or of counterrotating gas of mass fraction ξg<½, 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 ξ*+ξg ≈ ½, 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.
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