The Structure of Dark Matter Haloes in Hierarchical Clustering Models

We use a set of large cosmological N-body simulations to study the internal structure of dark matter haloes which form in scale-free models. We find that the radius r_178 corresponding to a mean interior overdensity of 178 accurately delineates the quasi-static halo interior from the surrounding infalling material, in agreement with the simple spherical collapse model. The interior velocity dispersion correlates with mass, again in good agreement with the spherical collapse model. Interior to the virial radius r_178, the spherical averaged density, circular velocity and velocity dispersion profiles are well fit by a simple 2-parameter analytic model proposed by Navarro etal (1995). This model has density going as 1/r at small radii, steepening to 1/r^3 at large radii, and fits our haloes to the resolution limit of the simulations. The two model parameters, scalelength and mass, are tightly correlated. Lower mass haloes are more centrally concentrated, and so have scalelengths which are a smaller fraction of their virial radius than those of their higher mass counterparts. This reflects the earlier formation times of low mass haloes. The haloes are moderately aspherical, with typical axial ratios 1:0.8:0.65 at their virial radii. These shapes are maintained by an anisotropic velocity dispersion tensor. The median value of the spin parameter is lambda=0.04, with a weak trend for lower lambda at higher halo mass. We also investigate how the halo properties depend on the algorithm used to identify them, using both friends-of-friends and spherical overdensity methods. We find that for groups selected at mean overdensities 100-400 by either method the properties are insensitive to how the haloes are selected, if the halo centre is taken as the position of the most bound particle