A warp is a smooth and moderate rise in a part of the external disk accompanied by a similar descent in the azimuthally opposed region, there remaining an integral sign or a mexican hat when galaxies are viewed edge-on. Concerning these, the information that can be extracted from the observations clearly depends on the assumed theoretical explanation of warps. A widely accepted theory of warps does not yet exist. Instead, different hypotheses have been proposed, some of which have survived the present observational tests.
Observations reveal a very large frequency of warped galaxies.
Statistical studies of warps have been made by Sanchez-Saavedra et al. In the first work an observed frequency of about 50 was derived. Warps are better observed in radio, as they largely extend far from the optical radius. On the other hand, a larger number of images are available in the optical. Given this large fraction of observed warps, the conclusion is that nearly all galaxies are warped.