Formation of spiral and elliptical galaxies in a CDM cosmogony

Abstract

The formation of galaxies in a CDM cosmogony is investigated by following the evolution of dark and baryonic matter and of the frequency- dependent spatially averaged radiation field. The gas is allowed to form stars which are independently treated as a fourth collisionless component. By supernovae, energy and metal enriched gas is returned to the interstellar medium. Two extreme cases, the formation of an isolated field galaxy and the merging of two gas rich proto spirals are considered the latter being used as a model for galaxy formation in a cluster. The isolated galaxy is assumed to interact with the surrounding matter via tidal fields only, resulting in a rotation with a spin parameter of about 0.08. The forming field galaxy shows the main properties of spiral galaxies: A metal rich, rotatianally flattened bulge, a metal poor anisotrope stellar halo and a exponetial disk of nearly solar composition. The spiral is embedded in a triaxial halo of dark matter. The stars of the bulge form from gas which is initially located in the largest maxima of the primordial density fluctuations, whereas the halo stars originate from gas accumulated in less pronounced maxima. Merging proto spirals evolve to objects which can be interpreted as luminous ellipticals . The metal rich anisotrope elliptical rotates only slowly. Depending on the geometry of the merger event sometimes counter rotating gas is observed.