A Unified Picture of Disk Galaxies where Bars, Spirals and Warps Result   from the Same Fundamental Causes
  Bars and spiral arms have played an important role as constraints on the dynamics and on the distribution of dark matter in the optical parts of disk galaxies. Dynamics linked to the dissipative nature of gas, and its transformation into stars provide clues that spiral galaxies are driven by dissipation close to a state of \textit{marginal stability} with respect to the dynamics in the galaxy plane. Here we present numerical evidences that warps play a similar role but in the transverse direction. N-body simulations show that typical galactic disks are also marginally stable with respect to a bending instability leading to typical observed warps. The frequent occurrence of warps and asymmetries in the outer galactic disks give therefore, like bars in the inner disks, new constraints on the dark matter, but this time in the outer disks. If disks are marginally stable with respect to bending instabilities, our models suggest that the mass within the HI disks must be a multiple of the detected HI and stars, i.e., disks must be heavier than seen. But the models do not rule out a traditional thick halo with a mass within the HI disk radius similar to the total disk mass. 
Daniel Pfenniger, Yves Revaz (Geneva Observatory, University of   Geneva)