A model for lopsided galactic disks
  Many disk galaxies are lopsided: their brightest inner parts are displaced from the center of the outer isophotes, or the outer contours of the HI disk. This asymmetry is particularly common in small, low-luminosity galaxies. We argue here that long-lived lopsidedness is a consequence of the disk lying off-center in the potential of the galaxy's extended dark halo, and spinning in a sense retrograde to its orbit about the halo center. The stellar velocity field predicted by our gravitational N-body simulations is clearly asymmetric. 
Stephen Levine (OAN/USNO) and Linda Sparke (U. Wisc.)