The angular momentum distribution in galactic halos

We have used N-body simulations to model the formation of individual galactic halos from scale-free density perturbations in universes dominated by cold, nondissipative dark matter. In well-mixed halos (i.e., halos without substructure), the angular momentum distribution is shown to have a systematic behavior with the power-law index n corresponding to that found for circular rotation curves. For a given n, the distribution of angular momentum has the same trend with radius and energy as that implied for a halo in which all the matter has its maximum possible angular momentum (coplanar, circular orbits). This behavior is expected if mixing and angular momentum transport during the relaxation of the halo are efficient in aligning the mean angular momentum of matter at different binding energies and the mean eccentricity of orbits is almost independent of binding energy. Dynamical mixing during the relaxation of the halo redistributes both angular momentum and binding energy in an orderly manner. The organized nature of the collapse means that (i) relaxation is not completely violent and (ii) the secondary infall paradigm, in its simplest form, needs to be modified to include the organizing effects of dynamical friction. We also show that the Mestel hypothesis is not consistent with the final collapsed state of halos but may be applicable to the collapse of the disks of spirals.