INSTABILITIES OF COUNTERROTATING STELLAR DISKS

Stellar disks having two equal populations of stars rotating in opposite directions are subject to several distinct instabilities. Here we focus on the six lowest-order modes: three within the disk plane and three bending modes. The form and vigor of the principal instabilities in any individual model vary with the balance between radial and azimuthal pressure and with disk thickness: thin radially hot models have disruptive axisymmetric bending instabilities (bell modes), while an in-plane lopsided instability is the most disruptive for cool models. All instabilities are weakened, but most rather slowly, by increasing disk thickness. The nonlinear evolution of the more unstable models takes an exotic variety of forms including lopsided and warped disks, permanent asymmetries about the initial plane, and pairs of counterrotating bars. Some of the radially hotter models acquire a thick subcomponent in the disk center resembling a bulge in appearance. Remarkably, however, the instabilities in a model having intermediate radial pressure cause rather mild changes and lead to an apparently stable, moderately thin, and almost axisymmetric disk. This model indicates that the SO galaxy NGC 4550, which has recently been discovered to possess two counterrotating disks, does not require large quantities of unseen mass to stablize it. This final state is also an unusual counterexample to the claim that instabilities should always thicken any nonrotating stellar system flatter than E7.