Formation of cuspy density profiles: A generic feature of collisionless gravitational collapse

Using the formalism of the spherical infall model, the structure of collapsed and virialized dark halos is calculated for a variety of scale-free initial conditions. In spite of the scale-free cosmological nature of the problem, the collapse of individual objects is not self-similar. Unlike most of previous calculations, the dynamics used here relies only on adiabatic invariants and not on self-similarity. This Letter focuses on the structure of the innermost part of the collapsed halos and addresses the problem of central density cusps. The slopes of density profiles at 1% of virial radius are calculated for a variety of cosmological models and are found to vary with the mass of the halos, power spectrum of density fluctuations, and Ohm (0),. The inner slopes range between r(-2.3) and r(-2) with the limiting case of r(-2) reached for the largest masses. The steep cusps found here correspond to the limiting case where all particles move on radial orbits. The introduction of angular momentum will make the density profile shallower. We expect this to resolve the discrepancy found between the calculated profiles and the ones found in high-resolution N-body simulations, where the exponent ranges from -0.5 to -1.5. The robust prediction here is that collisionless gravitational collapse in an expanding universe is expected to form density cusps and not halos with a core structure.