Stellar bar

An elongated nonaxisymmetric stellar structure in a galaxy, often approximated as a rotating perturbation of an axisymmetric or spherical gravitational potential. Its pattern angular velocity need not equal an individual star's orbital angular velocity. The Galactic bar is the particular example in the Milky Way.

Exchange of angular momentum between a stellar bar and a dark matter halo through orbital resonances. If the unperturbed halo distribution function depends only on energy and decreases with it, the resonant torque from a rotating perturbation is nonnegative for positive pattern angular velocity. Strict transfer requires nonzero resonant overlap. The reaction torque slows a rigid bar of constant positive moment of inertia; evolving bar structure and halo redistribution require updating the response.