The Warp and Spiral Arms of the Milky Way
  We examine the outer Galactic HI disk for deviations from the b=0 plane by constructing maps of disk surface density, mean height, and thickness. We find that the Galactic warp is well described by a vertical offset plus two Fourier modes of frequency 1 and 2, all of which grow with Galactocentric radius. A perturbation theory calculation demonstrates that the tidal influence of the Magellanic Clouds creates distortions in the dark matter halo, which combine to produce disk warp amplitudes comparable to the observations. We also construct a detailed map of the perturbed surface density of HI in the outer disk demonstrating that the Galaxy is a non-axisymmetric multi-armed spiral. Overdensities in the surface density are coincident with regions of reduced gas thickness. 
E.S. Levine, Leo Blitz, Carl Heiles, Martin Weinberg