Models for the Formation of Disc Galaxies

Abstract

Previously calculated models of the collapse of protogalaxies with rotational and axial symmetry are extended to galaxies characterized by both a spheroidal component and a disk component containing a substantial fraction of the total mass. The predicted disk-to-bulge ratio is found to depend mainly on the assumed rate of star formation and its time variation. It is shown that formation of the spheroidal component requires an early stage of rapid star formation, while formation of the disk requires a later stage of much slower star formation to allow the residual gas to settle into the disk before forming stars. The calculations suggest that the slow phase of star formation may involve relatively diffuse gas, the rapid phase may involve strongly clumped gas, and the degree of clumping may depend on the intensity of the turbulence or collisions within the gas. The models also predict stellar metallicity distributions and kinematic properties that are in qualitative agreement with those of the stellar populations in the Milky Way and other spirals, as well as a long time scale for the formation of the outer regions of the disk.