Velocity Dispersion

Random motions (orbits) of stars which support a self-gravitating body against collapse. Motions can be either ordered or random. The most prevalent form of ordered motion is rotation, in which the majority of stars orbit in the same direction. A galaxy with strong rotation is “rotation supported.” In contract, elliptical galaxies and the stellar halos and bulges of spiral galaxies show little or no rotation (as many stars orbiting in one direction as in another). Such a galaxy is “velocity dispersion” (or pressure) supported. Rotation can be measured by determining the galactic redshift at a number of positions across the galaxy. Measurement of velocity dispersion requires a measure of the velocity broadening of spectral lines.