The structure and dynamics of galaxies formed by cold dissipationless collapse

Abstract

We investigate the observable characteristics and phase-space structure of systems formed by dissipationless collapse. An extensive set of N-body simulations is described, starting from smooth initial conditions spanning a wide range of shapes and kinetic energies, both random and rotational. It is shown that collapses started from "cold" initial conditions (i.e., such that 2T/|U| ? 0.1, where T and U are the initial kinetic and potential energies, respectively) are qualitatively different from "warm" collapses, in the sense that their final shapes (roughly prolate, E5) are nearly uncorrelated with their initial shapes. We suggest that this is due to an "instability" similar to the radial-orbit instability in anisotropic equilibrium models. Warmer collapses tend to "remember" their initial shapes if nonrotating or become oblate if an appreciable amount of rotation is present. The minimum amount of rotation required to inhibit the bar instability corresponds to a spin parameter ? ? 0.10, larger than typical values for luminous elliptical galaxies. Cold initial conditions produce relaxed systems with projected density profiles that are well fitted by the r1/4 law; the quality of the fit increases as the initial temperature decreases, becoming essentially perfect for the coldest collapses that we consider. Although the same cold collapses that produce r1/4-law profiles exhibit the bar instability, these two phenomena have different origins: cold initial conditions evolved with an N-body code that enforces spherical symmetry develop realistic surface density profiles as well. The phase-space structure of these spherical systems is similar to that of models generated from the distribution function f(E, J2) oc |E|3/2 exp (-aE - bJ2), with a < 0 ("negative temperature") for the colder collapses. Our results suggest that (1) clumpy initial conditions are not required to produce a realistic final state via collapse, if the initial state is sufficiently cold; (2) if elliptical galaxies collapsed from cold and smooth initial configurations (and did not evolve significantly thereafter), they should be triaxial or prolate, with intrinsic flattenings of ?2:1.