GALACTIC SPIRAL SHOCKS WITH THERMAL INSTABILITY IN VERTICALLY STRATIFIED GALACTIC DISKS

Abstract

Galactic spiral shocks are dominant morphological features and believed to be responsible for substructure formation within spiral arms in disk galaxies. They can also contribute a substantial amount of kinetic energy to the interstellar gas by tapping the (differential) rotational motion. We use numerical hydrodynamic simulations to investigate dynamics and structure of spiral shocks with thermal instability in vertically stratified galactic disks, focusing on environmental conditions (of heating and the galactic potential) similar to the Solar neighborhood. We initially consider an isothermal disk in vertical hydrostatic equilibrium and let it evolve subject to interstellar cooling and heating as well as a stellar spiral potential. Due to thermal instability, a disk with surface density $\Sigma_0 \geq 6.7\Surf$ rapidly turns to a thin dense slab near the midplane sandwiched between layers of rarefied gas. The imposed spiral potential leads to a vertically curved shock that exhibits strong flapping motions in the plane perpendicular to the arm. The overall flow structure at saturation is comprised of arm, postshock expansion zone, and interarm regions that occupy typically 10\%, 20\%, and 70\% of the arm-to-arm distance, in which the gas resides for 15\%, 30\%, and 55\% of the arm-to-arm crossing time, respectively. The flows are characterized by transitions from rarefied to dense phases at the shock and from dense to rarefied phases in the postshock expansion zone, although gas with too-large postshock-density does not undergo this return phase transition, instead forming dense condensations. If self-gravity is omitted, the shock flapping drives random motions in the gas, but only up to $\sim 2-3 \kms$ in the in-plane direction and less than $2\kms$ in the vertical direction. Time-averaged shock profiles show... (abridged).