Coherent Stellar Motion in Galactic Spiral Arms by Swing Amplification
  We perform local $N$-body simulations of disk galaxies and investigate the
evolution of spiral arms. We calculate the time autocorrelation of the surface
density of spiral arms and find that the typical evolution timescale is
described by the epicycle period. We investigate the distribution of the
orbital elements of stars and find that in spiral arms the epicycle motions of
stars are in phase while the spatial distribution of the guiding center is
nearly uniform. These facts clearly show that the phase synchronization of the
epicycle motion takes place, which is theoretically predicted by the swing
amplification.
