RESONANT EXCITATION OF INTERNAL GRAVITY-WAVES IN CLUSTER COOLING FLOWS

Orbital motions of galaxies in X-ray clusters will resonantly excite internal gravity waves (g-waves) that propagate in the plasma. The resonance corresponds to a match between the local Brunt-Väisälä oscillation frequency in the plasma and the appropriate Fourier component of the galactic gravitational potential. Radiated internal waves have an inward group velocity that carries them to the central region of the flow where they become tightly wrapped, geometrically amplified, and eventually dissipated. Waves transport energy and angular momentum, but probably not in amounts large enough to grossly alter the structure of a cooling flow, should one be present. All physical flow variables are finite at the resonance, and all energy and angular momentum deposited by a galaxy is carried inward by the waves. The energy lost from the orbital motion of the galaxy to internal waves is similar to what would be found from the use of the collisionless dynamical friction equation. Nonlinear density and velocity fluctuations are likely to result from wave amplitude growth in the central regions of the flow, and a connection with the formation of emission-line filaments is a possibility. On the other hand, the existence of a finite group velocity implies that linear fluctuations are likely to be dispersed more rapidly than cooling can promote any thermal time scale instability that may formally be present in the gas.