We use hydrodynamical simulations to investigate the response of geometrically thin , self-gravitating , singular isothermal disks of gas to imposed rigidly rotating spiral potentials .
By minimizing reflection-induced feedback from boundaries , and by restricting our attention to models where the swing parameter X \sim 10 , we minimize the swing amplification of global normal modes even in models where Toomre ’ s Q _ { g } \sim 1 - 2 in the gas disk .
We perform two classes of simulations : short-term ones over a few galactic revolutions where the background spiral forcing is large , and long-term ones over many galactic revolutions where the spiral forcing is considerably smaller .
In both classes of simulations , the initial response of the gas disk is smooth and mimics the driving spiral field .
At late times , many of the models evince substructure akin to the so-called branches , spurs , and feathers observed in real spiral galaxies .
We comment on the parts played respectively by ultraharmonic resonances , reflection off internal features produced by nonlinear dredging , and local , transient , gravitational instabilites within spiral arms in the generation of such features .
Our simulations reinforce the idea that spiral structure in the gaseous component becomes increasingly flocculent and disordered with the passage of time , even when the background population of old disk stars is a grand-design spiral .
We speculate that truly chaotic behavior arises when many overlapping ultraharmonic resonances develop in reaction to an imposed spiral forcing that has itself a nonlinear , yet smooth , wave profile .