Dark Halos: The Flattening of the Density Cusp by Dynamical Friction

N-body simulations and analytical calculations of gravitational collapse in an expanding universe predict that halos should form with a diverging inner density profile, the cusp. There are some observational indications that the dark matter distribution in galaxies might be characterized by a finite core. This `core catastrophe' has prompted attenpts to seek alternatives to CDM cosmogony. It is shown here that the discrepancy between theory and observations can be very naturally resolved within the standard CDM model, provided that gas is not initially smoothly distributed in the dark matter halo, but rather is concentrated in clumps of mass $\geq 10^8 M_\odot$. Dynamical friction acting on these lumps moving in the background of the dark matter particles, dissipates the clumps orbital energy and deposits it in the dark matter. Using a semi-analytical model it is shown that dynamical friction provides a strong enough drag, and that the available orbital energy of the clumps is sufficient to heat the halo and turn the primordial cusp into a finite, non-diverging core.