Self-Perpetuating Spiral Arms in Disk Galaxies
  The causes of spiral structure in galaxies remain uncertain. Leaving aside
the grand bisymmetric spirals with their own well-known complications, here we
consider the possibility that multi-armed spiral features originate from
density inhomogeneities orbiting within disks. Using high-resolution N-body
simulations, we follow the motions of stars under the influence of gravity, and
show that mass concentrations with properties similar to those of giant
molecular clouds can induce the development of spiral arms through a process
termed swing amplification. However, unlike in earlier work, we demonstrate
that the eventual response of the disk can be highly non-linear, significantly
modifying the formation and longevity of the resulting patterns. Contrary to
expectations, ragged spiral structures can thus survive at least in a
statistical sense long after the original perturbing influence has been
removed.
