Transient spirals as superposed instabilities
  We present evidence that recurrent spiral activity, long manifested in
simulations of disk galaxies, results from the super-position of a few
transient spiral modes. Each mode lasts between 5 and 10 rotations at its
corotation radius where its amplitude is greatest. The scattering of stars as
each wave decays takes place over narrow ranges of angular momentum, causing
abrupt changes to the impedance of the disk to subsequent traveling waves.
Partial reflections of waves at these newly created features, allows new
standing-wave instabilities to appear that saturate and decay in their turn,
scattering particles at new locations, creating a recurring cycle. The spiral
activity causes the general level of random motion to rise, gradually
decreasing the ability of the disk to support further activity unless the disk
contains a dissipative gas component from which stars form on near-circular
orbits. We also show that this interpretation is consistent with the behavior
reported in other recent simulations with low mass-disks.
