A two-parameter scheme for the evolution of symmetrical galaxies

A general evolutionary scheme for axisymmetric rotationally supported equilibrium models for galaxies is described. The scheme consists of an expansion phase, a violent relaxation collapse phase, a virialization phase, and a contraction phase. The initial mean density and radius for both galaxy and component clouds act as physical parameters that determine the characteristics of the system at maximum expansion, together with the total amount of angular momentum acquired during the expansion phase. The main physical parameters at virialization are then completely specified when the initial distribution of the clouds inside the galaxy is assigned and the constants appearing in it are derived by normalization with the observed data. Two limiting cases are considered for the contraction phase following virialization: either the star component already present at virialization is completely neglected or it is thought to contract as the gas component.