Galactic Warps in Triaxial Halos
  We study the behaviors of galactic disks in triaxial halos both numerically
and analytically to see if warps can be excited and sustained in triaxial
potentials. We consider the following two scenarios: 1) galactic disks that are
initially tilted relative to the equatorial plane of the halo (for a
pedagogical purpose), and 2) tilted infall of dark matter relative to the
equatorial plane of the disk and the halo. With numerical simulations of
100,000 disk particles in a fixed halo potential, we find that in triaxial
halos, warps can be excited and sustained just as in spherical or axisymmetric
halos but they show some oscillatory behaviors and even can be transformed to a
polar-ring system if the halo has a prolate-like triaxiality. The
non-axisymmetric component of the halo causes the disk to nutate, and the
differential nutation between the inner and outer parts of the disk generally
makes the magnitude of the warp slightly diminish and fluctuate. We also find
that warps are relatively weaker in oblate and oblate-like triaxial halos, and
since these halos are the halo configurations of disk galaxies inferred by
cosmological simulations, our results are consistent with the fact that most of
the observed warps are quite weak. We derive approximate formulae for the
torques exerted on the disk by the triaxial halo and the dark matter torus, and
with these formulae we successfully describe the behaviors of the disks in our
simulations. The techniques used in deriving these formulae could be applied
for realistic halos with more complex structures.
