The formation of molecular clouds in spiral galaxies

ABSTRACT We present Smoothed Particle Hydrodynamics (SPH) simulations of molecular cloudformation in spiral galaxies. These simulations model the response of a non-self-gravitating gaseous disk to a galactic potential. The spiral shock induces high densitiesin the gas, and considerable structure in the spiral arms, which we identify as molec-ular clouds. We regard the formation of these structures as due to the dynamics ofclumpy shocks, which perturb the flow of gas through the spiral arms. In addition, thespiral shocks induce a large velocity dispersion in the spiral arms, comparable with themagnitude of the velocity dispersion observed in molecular clouds. We estimate theformation of molecular hydrogen, by post-processing our results and assuming the gasis isothermal. Provided the gas is cold (T 6 100 K), the gas is compressed sufficientlyin the spiral shock for molecular hydrogen formation to occur in the dense spiral armclumps. These molecular clouds are largely confined to the spiral arms, since mostmolecular gas is photodissociated to atomic hydrogen upon leaving the arms.Key words: galaxies: spiral – hydrodynamics – ISM: clouds – ISM: molecules – stars:formation