The Collapse of Our Galaxy and the Formation of the Galactic Disk

The self-regulated chemical and dynamical evolution of an initially hot, gaseous protodisk is examined in detail, taking into account both star formation and heating and cooling processes in a multicomponent interstellar medium. It is shown that one can understand the observed structure of the Galactic disk in a natural way as a consequence of the gravitational settling of the protodisk. During the first 4-6 x 10 exp 9 yr, stars form in a thick disk and 'old thin disk' component, with kinematical and chemical properties which are in agreement with the observations. The restriction of star formation to a 'young thin disk' is delayed by 4-6 x 10 exp 9 yr. These models show a strong dependence of the evolutionary time scale on the total surface density Sigma, which implies that the galactic disk formed from inside out. Furthermore, it is found that no thin disk component forms for Sigma not greater than 10 solar masses/sq pc.