Spiral density wave
= Spiral density wave
A collective perturbation of <surface density> and <gravitational potential> in a <galactic disk>. Material can cross its spiral-shaped crests; the pattern angular speed need not equal the local material angular speed. A tightly wound fluid approximation has $(\omega-m\Omega)^2=\kappa^2+c_s^2k^2-2\pi G\Sigma|k|$, while a collisionless stellar disk requires its own response function.