Scale Lengths in Dark Matter Halos

Abstract

We investigate a hypothesis regarding the origin of the scale length in halos formed in cosmological N-body simulations. This hypothesis can be viewed as an extension of an earlier idea put forth by Merritt and Aguilar. Our findings suggest that a phenomenon related to the radial orbit instability is present in such halos and is responsible for density profile shapes. This instability sets a scale length at which the velocity dispersion distribution changes rapidly from isotropic to radially anisotropic. This scale length is reflected in the density distribution as the radius at which the density profile changes slope. We have tested the idea that radially dependent velocity dispersion anisotropy leads to a break in density profile shape by manipulating the input of a semianalytic model to imitate the velocity structure imposed by the radial orbit instability. Without such manipulation, halos formed are approximated by single power-law density profiles and isotropic velocity distributions. Halos formed with altered inputs display density distributions featuring scale lengths and anisotropy profiles similar to those seen in cosmological N-body simulations.