The structure and dynamics of galaxies formed by cold dissipationless collapse

Abstract

The observable characteristics and phase-space structure of systems formed by dissipationless collapse are investigated. An extensive set of N-body simulations is described; and three-dimensional and spherical collapses, their shapes, and dynamics are examined. It is demonstrated that collapses started from cold initial conditions are quantitatively different from warm collapses, in the sense that their final shapes are nearly uncorrelated with their initial shapes. It is suggested that this phenomenon is due to an instability similar to the radial-orbit instability in anisotropic equilibrium models. The results show that clumpy initial conditions are not required to produce a realistic final state via collapse, if the initial state is sufficiently cold; and that if elliptical galaxies collapsed from cold and smooth initial configurations (and did not evolve significantly thereafter), they should be triaxial or prolate, with intrinsic flattening of approximately 2:1. 24 refs.