ABSTRACT

The large-scale dynamics of the interstellar medium in spiral galaxies is significantly determined by molecular cloud physics. Inelastic cloud-cloud collisions, coupled with gravitational instabilities generated by the gas cooling, give rise to a net flux of angular momentum which is equivalent to a (gravitational) “viscosity”. By using N–body model calculations of a self-gravitating disk we study the efficiency of collisions coupled with gravitational waves, and its importance for large-scale mass accretion in disk galaxies.

THE IDEA

The mass of the interstellar medium of galaxies is essentially included in an ensemble of clouds, whose physics on a small-scale dominates the dynamical behaviour. In a non-axisymmetric potential, the gas clouds are submitted to gravitational torques, which transfer angular momentum outwards, inducing an accumulation of gas in the central regions. Combes and Gèrin (1985) developed a collision model for the interstellar medium, including a mass spectrum of molecular clouds. If on the other hand, differential rotation is sufficient, the gas is submitted to viscous torques, which also accumulates gas in the central regions (Lesch et al. 1990). It is the aim of our investigation to connect these different approaches, by identifying the angular momentum transport in a non-axisymmetric potential with a viscosity. A study of the exchange of angular momentum and energy between clouds will provide an understanding of the nature of the gas “viscosity”.