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We propose a new dynamical picture of galactic stellar and gas spirals, basedon hydrodynamic simulations in a `live' stellar disk. We focus especially onspiral structures excited in a isolated galactic disk without a stellar bar.Using high-resolution, 3-dimensional N-body/SPH simulations, we found that thespiral features of the gas in galactic disks are formed by essentiallydifferent mechanisms from the galactic shock in stellar density waves. Thestellar spiral arms and the interstellar matter on average corotate in agalactic potential at any radii. Unlike the stream motions in the galacticshock, the interstellar matter flows into the local potential minima withirregular motions. The flows converge to form dense gas clouds/filaments nearthe bottom of the stellar spirals, whose global structures resemble dust-lanesseen in late-type spiral galaxies. The stellar arms are non-steady; they arewound and stretched by the galactic shear, and thus local densities of the armchange on a time scale of ~ 100 Myrs, due to bifurcating or merging with otherarms. This makes the gas spirals associated with the stellar arms non-steady.The association of dense gas clouds are eventually dissolved into inter-armregions with non-cirucular motions. Star clusters are formed from the cold,dense gases, whose ages are less than ~30 Myrs, and they are roughly associatedwith the background stellar arms without a clear spatial offset between gasspiral arms and distribution of young stars.

Interplay between Stellar Spirals and the ISM in Galactic Disks