Warped galactic disks and optical projection effects

Direct and indirect evidence for global warping in galactic disks is reviewed. The existing data suggest that inversion symmetric warping is common in spiral galaxies, that it can occur throughout the inner and outer disk regions, and that it can involve stellar, dust, and gas components. The amplitude of the warping tends to be proportional to the radial distance from the galactic center and varies from instances wherein the disk is essentially flat to cases wherein the z-axis displacement is at least 30 percent of the galactocentric distance. Projection effects that are inherent features of warped optical disks are discussed. (Author)