In some external edge-on spiral galaxies, such as NGC 891, 4244, 4565 and 5907, the observed light distribution exhibits a sharp truncation in the outer parts (e.g., Casertano 1983, hereafter C83). As a rule, such galaxies are known to be warped in their gas disk, the deformation starting at about their optical cut-off radius. These facts suggest a truncation in the disk density distribution. Moreover, the rotation velocity of these galaxies starts decreasing at about the same radius, which can also be considered as the "signature" of truncation (see C83).
Both giant spiral galaxies in the Local Group, the Milky Way and M31, have a dip in the average rotation velocity which can be produced by a truncation of the galactic disk. Ninković & Petrovskaya (1992) constructed from Haud’s (1981) rotation curve for M31 a model including, like C83’s for NGC 5907, a truncated exponential disk and a halo. The truncation distance was found to be close to the radius beyond which warping of the hydrogen layer of M31 begins.