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 { M _ { \odot } pc ^ { -2 } } 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 { km s ^ { -1 } } in the in-plane direction and less than 2 { km s ^ { -1 } } in the vertical direction .
Time-averaged shock profiles show that the spiral arms in stratified disks are broader and less dense compared to those in unstratified models , and that the vertical density distribution is overall consistent with local effective hydrostatic equilibrium .
Inclusion of self-gravity increases the dense gas fraction by a factor \sim 2 and raises the in-plane velocity dispersion to \sim 5 - 7 { km s ^ { -1 } } .
When the disks are massive enough , with \Sigma _ { 0 } \geq 5 { M _ { \odot } pc ^ { -2 } } , self-gravity promotes formation of bound clouds that repeatedly collide with each other in the arm and break up in the postshock expansion zone .