Simulations of the effect of spiral arms on the cloud-ensemble velocity dispersion

Artikel i vetenskaplig tidskrift, 1991

N-body simulations are used to study the effect of spiral heating, viscous heating, and cooling by inelastic collisions on the molecular cloud ensemble velocity dispersion. The simulations include inelastic collisions between clouds as well as long-range gravitational forces. The theory for heating and cooling of the cloud population is described, and the numerical code and the galaxy model are examined. Spiral arms not only heat the cloud population, they also cause increased cooling due to the large number of cloud-cloud collisions in the arms. It is found that the heating dominates, in that a stronger spiral potential leads to a higher velocity dispersion. The spiral potential is stronger for smaller values of Q(star). The steady-state cloud-ensemble velocity dispersion is found to be constant with the radius. Its value for a typical simulation of a multiple-arm spiral is slightly smaller than the value observed in the Galaxy. The stellar velocity dispersion corresponds to a radius-independent Q(star), for multiple-arm galaxies regardless of the initial radial dependence.