Implications of collisionally supported giant molecular clouds for spiral galactic structure and massive star formation

A study has been made of the environment experienced by ballistically moving giant molecular clouds (GMCs) as they encounter a hydrodynamically flowing interstellar medium. The hydrodynamically flowing gas is assumed to consist of an ensemble of discrete small clouds. GMCs are assumed to be born in spiral galactic shock fronts from which they are ejected at postshock velocities. The mechanism for the support of GMCs suggested by Bash, Hausman, and Papaloizou is invoked to determine the fate of these clouds (i.e., lifetime and star-forming capability). According to that model, support occurs when the energy loss through decay of turbulence in the clouds is balanced by energy input via collisions with smaller clouds. Additionally, we assume that an excessive energy input will destroy a GMC and that an unsupported cloud will collapse to form massive stars and H II regions. if the galactic distributions of small colliding clouds are like those observed for H I, the model predictions are in agreement with several observational characteristics of the Galaxy and M8 t. Radial rates of star formation (the H II region distributions) are reproduced for both galaxies, as are the H II region locations relative to the spiral shocks in M8 1. We also predict CO and H II region velocity-longitude diagrams for the Galaxy. No such agreement with observations is seen when the small clouds are assumed to be distributed in a "molecular ring."