On Universal Halos and the Radial Orbit Instability

The radial orbit instability drives the density profile of a collapsing nearly spherical density perturbation toward the universal form found in cosmological simulations. This conclusion, first noted by Huss and coworkers, is explored in detail through a series of numerical experiments involving isolated halos. Simulations are run with and without the nonradial forces responsible for the instability. In the absence of the instability, the density profile is a pure power law, the differential energy distribution is close to a Boltzmann distribution, and the orbits are radially biased. The instability transforms the density profile to the NFW form, induces an energy cutoff at high binding energy, and isotropizes the orbits near the center of the system. New insights into the underlying physics of the radial orbit instability are presented.