Stellar dynamics around transient co-rotating spiral arms
  Spiral density wave theory attempts to describe the spiral pattern in spiral
galaxies in terms of a long-lived wave structure with a constant pattern speed
in order to avoid the winding dilemma. The pattern is consequently a rigidly
rotating, long-lived feature. We run an N-body/SPH simulation of a Milky
Way-sized barred disk, and find that the spiral arms are transient features
whose pattern speeds decrease with radius, in such a way that the pattern speed
is almost equal to the rotation curve of the galaxy. We trace particle motion
around the spiral arms. We show that particles from behind and in front of the
spiral arm are drawn towards and join the arm. Particles move along the arm in
the radial direction and we find a clear trend that they migrate toward the
outer (inner) radii on the trailing (leading) side of the arm. Our simulations
demonstrate that at all radii where there is a co-rotating spiral arm the
particles continue to be accelerated (decelerated) by the spiral arm for long
periods, which leads to strong migration.
