DYNAMICS OF MOLECULAR CLOUDS ON THE GALACTIC SCALE .1. A MODEL WITH AXIAL SYMMETRY

We have carried out an extensive investigation into the dynamics of th e molecular clouds in the disk of the Galaxy. We have used both comput ational methods and physical arguments to try to understand how the en semble of molecular clouds interacts, how the clouds are affected by t he gravitational field of the Galaxy and also the circumstances under which they can aggregate into giant molecular clouds (GMC's). The dyna mical model is three dimensional and consists of 120,000 spherical clo uds, each having a mass of 10(4) M.. It allows for the mutual gravitat ion between clouds, up to a cut-off distance; when two clouds collide they rebound, with a specified coefficient of restitution e. We have a lso developed a physically more realistic model for a cloud, supported by a magnetic field, and used it to select a suitable range of values for e. Our first paper deals with the case of an axially symmetrical galaxy. The clouds are distributed initially in a disk extending 100 p c on either side of the Galactic plane. As it evolves the system of cl ouds loses energy, and the disk grows thinner at a rate which depends on the value of e. GMC's start to form once the disk is thin enough. W e believe this result to be valid more generally, and that it holds al so in models with spiral structure.