Interplay between Stellar Spirals and the ISM in Galactic Disks
  We propose a new dynamical picture of galactic stellar and gas spirals, based on hydrodynamic simulations in a `live' stellar disk. We focus especially on spiral structures excited in a isolated galactic disk without a stellar bar. Using high-resolution, 3-dimensional N-body/SPH simulations, we found that the spiral features of the gas in galactic disks are formed by essentially different mechanisms from the galactic shock in stellar density waves. The stellar spiral arms and the interstellar matter on average corotate in a galactic potential at any radii. Unlike the stream motions in the galactic shock, the interstellar matter flows into the local potential minima with irregular motions. The flows converge to form dense gas clouds/filaments near the bottom of the stellar spirals, whose global structures resemble dust-lanes seen in late-type spiral galaxies. The stellar arms are non-steady; they are wound and stretched by the galactic shear, and thus local densities of the arm change on a time scale of ~ 100 Myrs, due to bifurcating or merging with other arms. This makes the gas spirals associated with the stellar arms non-steady. The association of dense gas clouds are eventually dissolved into inter-arm regions with non-cirucular motions. Star clusters are formed from the cold, dense gases, whose ages are less than ~30 Myrs, and they are roughly associated with the background stellar arms without a clear spatial offset between gas spiral arms and distribution of young stars. 
Keiichi Wada, Junichi Baba, Takayuki R. Saitoh
astro-ph.GA