Pattern speeds in the Milky Way

Understand

A brief review is given of different methods used to determine the pattern speeds of the Galactic bar and spiral arms.

• The Galactic bar rotates rapidly, with corotation about halfway between the Galactic center and the Sun, and outer Lindblad resonance not far from the solar orbit, R0.
• The Galactic spiral arms currently rotate with a distinctly slower pattern speed, such that corotation is just outside R0.
• Both structures therefore seem dynamically decoupled.