  For a decade, N-body simulations have revealed a nearly universal dark matter
density profile, which appears to be robust to changes in the overall density
of the universe and the underlying power spectrum. Despite its universality,
the physical origin of this profile has not yet been well understood.
Semi--analytic models by Barnes et al. (2005) have suggested that the density
structure of dark matter halos is determined by the onset of the radial orbit
instability (ROI). We have tested this hypothesis using N-body simulations of
collapsing dark matter halos with a variety of initial conditions. For
dynamically cold initial conditions, the resulting halo structures are triaxial
in shape, due to the mild aspect of the instability. We examine how variations
in initial velocity dispersion affect the onset of the instability, and find
that an isotropic velocity dispersion can suppress the ROI entirely, while a
purely radial dispersion does not. The quantity sigma^2/vc^2 is a criterion for
instability, where regions with sigma^2/vc^2 <~1 become triaxial due to the ROI
or other perturbations. We also find that the radial orbit instability sets a
scale length at which the velocity dispersion changes rapidly from isotropic to
radially anisotropic. This scale length is proportional to the radius at which
the density profile changes shape, as is the case in the semi--analytic models;
however, the coefficient of proportionality is different by a factor of ~2.5.
We conclude that the radial orbit instability is likely to be a key physical
mechanism responsible for the nearly universal profiles of simulated dark
matter halos.
