The above results refer to the gas, because emission lines are relatively easy to observe. As will be shown, this difference is due to the velocity dispersion of the stellar component. Unlike some elliptical galaxies, bulges generally have well-aligned rotation and minor axes. In normal bulges, rotation velocity decreases smoothly with increasing height above the disk. 4.8 Warps The same observations which establish the presence of dark halos also show that the outer regions of disk galaxies are often warped.
Warps are very common.
While elliptical galaxies obey a number of different parameter correlations, rather few such relations are known for spiral galaxies. This is very similar to the Faber-Jackson relation for elliptical galaxies, which states that luminosity scales as the fourth power of the central velocity dispersion. Nonetheless, the TF relation is very useful for measuring distances to disk galaxies. References Binney, J. 1992, Ann. Briggs, F. 1990, Ap.