Continuum models for gas in disturbed galaxies. I - Model equations

Model hydrodynamic equations to describe the macroscopic nonequilibrium behavior of galactic gas flows are presented. The model pictures galactic gas as consisting primarily of clouds which are embedded in an ambient intercloud medium and a stellar gravitational field. The criteria for the validity of a continuum model of such a discrete cloud system are examined and it is shown that the gas in the Galaxy satisfies these criteria. A kinetic equation for the evolution of the clouds in position-velocity-mass phase space is used to derive moment equations for the number density of clouds, their mean mass, mean flow velocity, velocity dispersion, and stress tensor. A number of approximations to close the equations are critically discussed. Source terms in the model equations represent the effects of cloud collisions, spontaneous cloud creation from the intercloud medium, cloud disruption due to internal star formation activity, and acceleration and formation of clouds by energetic stellar processes. Physically reasonable parameterized forms for these source terms are presented, and it is shown that most of these parameters can be constrained by observational or theoretical considerations.