The structure of cold dark matter halos

Abstract

The density profiles and shapes of dark halos are studied using the results of N-body simulations of the gravitational collapse of density peaks. The simulations use from 30,000 to 300,000 particles, which allow density profiles and shapes to be well resolved. The core radius of a typical dark halo is found to be no greater than the softening radius, epsilon = 1.4 kpc. The density profiles can be fitted with an analytical model with an effective power law which varies between -1 in the center to -4 at large radii. The dark halos have circular velocity curves which behave like the circular velocity contribution of the dark component of spiral galaxies inferred from rotation curve decompositions. The halos are strongly triaxial and very flat, with (c/a) = 0.50 and (b/a) = 0.71. There are roughly equal numbers of dark halos with oblate and prolate forms. The distribution of ellipticities in projection for dark halos reaches a maximum at epsilon = 0.5, in contrast to the ellipticity distribution of elliptical galaxies, which peaks at epsilon = 0.2.