Models for the formation of disc galaxies

Previous calculations of the collapse of protogalaxies with rotation and axial symmetry are here extended to galaxies possessing both a spheroidal component, and a disc component containing a substantial fraction of the total mass. The predicted disc:bulge ratio depends mostly on the assumed star formation rate and how it varies with time: the formation of a spheroidal component requires the early phase of rapid star formation, but the formation of a disc requires a later stage of much slower star formation which allows residual gas to settle to a disc before forming stars. The rapid mode of star formation may involve strongly clumped gas and the slow mode relatively diffuse gas, and the degree of clumping may depend on the strength of turbulence or collisions in the gas; the Hubble sequence can then be understood at least in part as resulting from differences in the initial density or velocity dispersion in protogalaxies. Other general predictions of the models include stellar metallicity distributions and kinematic properties that are in qualitative agreement with those of the stellar populations in our own and other spiral galaxies, and a long time scale for the formation of the outer part of the disc, implying a significant gas content and star formation rate even after 1010yr. (author)