FORMATION AND EVOLUTION OF THE DISK IN SPIRAL GALAXIES - VISCOUS MODELS WITHOUT INFALL

We study models for the evolution of the disc of spiral galaxies that include the effects of viscosity but assume no continuous infall of gas on to the disc. The time-scale for star formation is assumed to be of the same order as the time-scale for viscous redistribution of the mass and angular momentum in the disc. We confirm that these models are able to reproduce the observed radial distribution of gas and stars for present day spirals for a variety of model parameters and initial conditions. Furthermore, we generally find that the evolution of both the stellar and gas surface density profiles is approximately self-similar over at least 90 per cent of the history of the disc, and that the size of the initial gas disc is approximately equal to the present Holmberg radius for the galaxy. Implications of these results for the evolution of spiral structure and for the interpretation of damped Ly alpha-lines in the spectra of high-redshift QSOs are considered. A discussion of the possible physical mechanisms for the viscosity and the associated time-scales is also presented.