Instabilities of spiral shocks. II. A quasi-steady state in the multiphase inhomogeneous ISM

Galactic shocks ( Fujimoto; Roberts) are investigated using full three-dimensional hydrodynamic simulations that take into account the self-gravity of the ISM, radiative cooling, and star formation followed by energy feedback from supernovae. This is an essential improvement over previous numerical models, in which two-dimensional isothermal, non-self-gravitating gas is assumed. We find that the classic galactic shocks are unstable and transient, and they shift to a globally quasi-steady, inhomogeneous pattern due to the nonlinear development of instabilities in the disk. The spiral patterns consist of many giant molecular cloud-like dense condensations, but those local structures are not steady, and they evolve into irregular spurs in the interarm regions. Energy feedback from supernovae does not destroy the quasi-steady spiral arms; rather, it mainly contributes to the vertical motion and the structures of the ISM. The results and methods presented here are a starting point for a more consistent treatment of the ISM in spiral galaxies, in which the effects of magnetic fields, radiative transfer, chemistry, and dynamical evolution of a stellar disk are taken into account.