Appearance of co-existing spiral modes

The animation on the right shows the time evolution of the net overdensity of two superposed steady-amplitude, bi-symmetric spiral blobs (shown to the left) that turn at differing rates. The inner blob has the higher pattern speed. The blobs are just drawings, with no underlying dynamics.

The appearance of the time evolution of these two superposed steady "waves" is of a shearing transient pattern. The peak amplitude rises as the pattern "swings" from slighly leading to training and then decreases as the spiral becomes more tightly wrapped, before finally the pattern snaps and repeats. Thus simulations that exhibit recurrent transient spiral patterns, as they always do, could still be supporting long-lived underlying spirals.

I (Jerry Sellwood) first showed this animation at the "Stars Without Borders" Workshop, held in Medana, Slovenia on May 21, 2012. Please acknowledge the author if you download the animation.