Flyby Creation of Density Wave

Three nice examples of tidally induced two-armed density wave spiral arms.


The famous 1970s simulation of M51 by Toomre & Toomre. The flyby generates a strong m = 2 perturbation that induces a (temporary) strong two-armed density wave. The second column shows the flyby from the plane of the sky, with the sun to the left.

More recent simulation of a self-gravitating disk response to a flyby. The strong m = 2 perturbation generates a strong two-armed spiral. However, after a few rotations the arms get much weaker, so the arms are transient