In spite of being known now for many decades, warps are still a
puzzle. The observational situation and theoretical models to explain
the data are still evolving. In this talk I have tried to summarize
the situation as follows.
- Warps are a very common phenomenon. However, in addition to the
classic `grand-design' integral-sign warps, in many cases the warp is
asymmetric or even one-sided. Models for warps should therefore not
only address the symmetric, regular ones. Quite possibly there
is a good analogy here to spiral structure, which also displays varying
degrees of regularity.
- Normal modes (equilibrium configurations of a tilted, warped
disk precessing about the symmetry axis of a flattened halo) interact
strongly with the dark halo. As a consequence they are easily and
strongly damped, or, in special circumstances, excited.
- Satellite tides are generally too weak to produce warps of the
amplitudes observed. However the dark halo can under certain
circumstances respond to the satellite in a way that
significantly adds to the tidal perturbation on the disk. A useful
diagnostic for satellite tides as explanation for a warp is the
orientation of the warp with respect to the satellite orbit. In the
case of the LMC / Galaxy system, this orientation appears to be
different from predictions.
- Accretion of material can generate warps through a continual
change in the orientation of the halo symmetry plane, to which
different parts of the disk respond on different timescales. Such
models are yet to be investigated in full detail; in particular the
question of whether the observed frequency of asymmetric warps can be
reproduced by this kind of model may be a useful avenue to explore.