Galaxy dynamics studies how gravity organizes the motions of stars, gas and dark matter in galaxies. Doppler shifts of spectral lines measure motion along the line of sight; proper motions, where measurable, supply transverse motion. A rotation curve constrains the radial gravitational force, while stellar random motions constrain support in a galactic bulge or elliptical galaxy. The interpretation depends on geometry and on whether the observed material follows nearly circular orbits. Stellar bars and spiral arms exert torques, exchange angular momentum with the surrounding galactic disk and can drive gas inward. Thus the dynamics both measures the mass distribution and explains how that distribution changes.[ref]
Spirals in Galaxies by Jerry A. Sellwood and Karen L. Masters. Discussion of disk dynamics and secular changes driven by spiral structure
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