Velocity dispersion of the giant molecular clouds - a viscous origin

The energy source is proposed and the details of the acceleration mechanism for the random motion of the giant molecular clouds (GMCs) in the Galaxy are discussed. Specifically, it is proposed that gravitational scattering of the massive clouds off each other in the differentially rotating Galactic disk constitutes an effective gravitational viscosity, which causes an increase in the random kinetic energy of the GMCs at the expense of their ordered, rotational kinetic energy in the Galactic disk. The energy input due to this gravitational viscosity is calculated, and the other terms contributing to the energy balance of the GMCs are computed. These results are used to determine the steady state cloud velocity dispersion and its dependence on the cloud mass and radius and on the galactocentric radial distance. 30 references