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 setting of the protodisk. During the first 4-6 x 10(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(9) yr. Our 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, we find that no thin disk component forms for SIGMA less-than-or-equal-to 10 M. pc-2.