The Equilibrium Structure of CDM Halos

Dark-matter (DM) halos are the scaffolding around which galaxies and clusters are built. They form when the gravitational instability of primordial density fluctuations causes regions which are denser than average to slow their cosmic expansion, recollapse, and virialize. Understanding the equilibrium structure of these halos is thus a prerequisite for understanding galaxy and cluster formation. Numerical N-body simulations of structure formation from Gaussian-random-noise initial conditions in the Cold Dark Matter (CDM) universe find a universal internal structure for halos. Objects as different in size and mass as dwarf spheroidal galaxies and galaxy clusters are predicted to have halos with the same basic structure when properly rescaled, independent of halo mass, of the shape of the power spectrum of primordial density fluctuations, and of the cosmological background parameters. This remarkable universality is a fundamental prediction of the CDM model, but our knowledge is limited to the “empirical” N-body simulation results, with little analytical understanding. We summarize here our attempts to fill this gap, in an effort to derive and give physical insight to the numerical results and extend them beyond the range of numerical simulation: (1) Simulated halos which form from highly simplified initial conditions involving gravitational instability in a cosmological pancake show that many of the universal properties of CDM halos are generic to cosmological gravitational collapse and do not require Gaussian-random-noise density fluctuations or hierarchical clustering. (2) A fluid approximation derived from the Boltzmann equation yields an analytical theory of halo dynamics which can explain many of the N-body results if the complex mass assembly history of individual halos is approximated by continuous spherical infall. The universal mass growth history reported for CDM N-body halos corresponds to a time-varying infall rate which self-consistently determines the shape of the equilibrium halo profile and its evolution without regard for the complicated details of the merger process. (3) The first fully-cosmological, similarity solutions for halo formation in the presence of collisionality provide an analytical theory of the effect of the Department of Astronomy, University of Texas, Austin, 78712, US Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada Département de physique, de génie physique et d’optique, Université Laval, Sainte-Foy, QC G1K 7P4, Canada