Viscous evolution of self-gravitating galactic disks within a dark halo

Abstract

Viscous evolution of thin and star-forming galactic disks within a massive dark halo is investigated by taking into account the self- gravity. We assume that the well-known gravitational instability of a self-gravitating disk generates turbulence (as well as star formation). The disk is assumed to be in nearly centrifugal equilibrium under the action of viscosity, just like an accretion disk. Such a disk is found to be unstable to spatial oscillations of physical variables owing to the action of viscosity and self-gravity in the inner part of the disk where rotation is nearly uniform. Suppressing the instability by taking into account the turbulent pressure, we have numerically followed the viscous evolution of star- forming self-gravitating disks until most of the gas has been converted to stars. Exponential distributions of the surface density of the stars are produced as a result of viscous evolution with star formation. Moreover, it is found that the viscous evolution creates flat rotation curves extending from the outer part where dark halo dominates to the inner part where self-gravity dominates, just as observations indicate. Observed variety in the shape of the rotation curves near the center among galaxies seems to be reproduced by changing the efficiency of star formation or initial angular momentum of the disks.