Dark halos formed via dissipationless collapse. I - Shapes and alignment of angular momentum

We use N-body simulations on highly parallel supercomputers to study the structure of Galactic dark matter halos. The systems form by gravitational collapse from scale-free and more general Gaussian initial density perturbations in an expanding 400 Mpc^-3^ spherical slice of an Einstein-deSitter universe. We use N ~ 10⁶^ and a force softening ɛ = 5 kpc in most of our models. We analyze the structure and kinematics of the ~10²^ largest relaxed halos in each of 10 separate simulations. A typical halo is a triaxial spheroid which tends to be more often prolate than oblate. These shapes are maintained by anisotropic velocity dispersion rather than by angular momentum (spin parameter λ ~ 0. 05). Nevertheless, there is a significant tendency for the total angular momentum vector to be aligned with the minor axis of the density distribution. These features (halo shape and orientation with respect to angular momentum) are maintained throughout the virialized portion of the halo. We have also studied the sensitivity of the shapes to the smoothing length ɛ and find that halos tend to become less prolate at small radii for ɛ = 1 kpc. Numerical and astrophysical consequences of this result are briefly considered. Subject Headings: galaxies: formation - galaxies: kinematics and dynamics - methods: numerical