Bending instabilities at the origin of persistent warps: a new   constraint on dark matter halos
  A substantial fraction of the warps in spiral galaxies may result from bending instabilities if the disks are essentially self-gravitating. With N-body simulations, we show that galaxies with self-gravitating disks as thick as HI disks are subject to bending instabilities generating S-shaped, U-shaped or asymmetric warps. S-shaped warps persist during several rotations and keep the line of node straight. The warp amplitudes generated by bending instabilities remain however modest. Other factors must be invoked for extreme warped disks. However, bending instabilities can account for most of the cases reported in optical surveys, where the warp angle is generally less than $5^\circ$. This mode of warping is very sensitive to the disk flattening. It also constrains the fraction of dark matter distributed in the disk and in the dark halo. 
Yves Revaz, Daniel Pfenniger (Geneva Observatory)