THE DAMPING AND EXCITATION OF GALACTIC WARPS BY DYNAMICAL FRICTION

We investigate the dynamical interaction of galactic warps with the surrounding dark matter halo, using analytic perturbation theory. A pressing warp induces a density wake in the collisionless dark matter, which acts back on the original warp, transferring energy and angular momentum between the warp and halo (dynamical friction). In most cases dynamical friction damps the warp, but in unusual circumstances (a halo that rotates in the same direction as the precession of the warp, or a warp in the equatorial plane of an axisymmetric prolate halo) friction can excite a warp. The damping/excitation time is usually short compared with the Hubble time for realistic systems. Thus most warps cannot be primordial; they must be recently or continuously excited.