Models of the long-term evolution of the Galactic disk with viscous flows and gas infall

Radial gas flows induced by viscosity may significantly redistribute the gas component in galactic disks. On the condition that the timescale of the viscous transport is comparable to the star formation timescale, this process naturally explains the observed exponential stellar density profiles in disks independent of the density profiles of the initial disks. We discuss models for the chemical and dynamical evolution of the Galactic disk which combine viscous radial gas flows with infall of external gas onto the disk, and infall-induced radial gas flows. The main goal of this study is the confrontation of model results with a large set of observational constraints from the Galactic disk. The star formation rate and the viscosity in the interstellar medium are parametrised as usual in modelling the evolution of a viscous star forming disk. The element abundance evolution for oxygen and iron is driven by supernovae of types ia and ii. Chemical inhomogeneities are taken into account for the interstellar medium at given time and Galactocentric distance. In the result of a large number of model computations, we found several distinct models which provide an acceptable fit to most of the observational data. However, the models are not appropriate to fit all observed details, partly due to the simple description of the star formation rate. We discuss the evolution of radial abundance gradients. Although viscous models generally produce gradients, their values are typically only −0.03 dex kpc−1 if we adopt the initial gas density distribution of an isothermal sphere. Several processes are discussed which can enhance the gradients. Viscous evolution generates a strong mass concentration in the inner few kpc. We compare the properties of the central stellar population in the models with the Galactic bulge and discuss the possible relationship between viscous flows and bulge formation. Furthermore, we propose that the observed radial gas density distribution in the innermost disk may be produced by strongly enhanced viscosity in that region, e.g. due to a central bar.