  We investigate the dynamical interaction of galactic warps with the
surrounding dark matter halo, using analytic perturbation theory. A precessing
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 (such as 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 to the Hubble time for realistic systems. Thus
most warps cannot be primordial; they must be maintained by some ongoing
excitation mechanism.
