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

Abstract

N-body simulations and analytical calculations of the 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 a search for alternatives to the 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 $\geq 0.01 %$ the total mass of the system. 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 Monte-Carlo simulations, it is shown that the dynamical friction provides a strong enough drag, and that with realistic baryonic mass fractions, the available orbital energy of the clumps is sufficient to heat the halo and turn the primordial cusp into a finite, non-diverging core --- overcoming the competing effect of adiabatic contraction due to gravitational influence of the shrinking baryonic component. Depending on the initial conditions, the total density distribution may either become more or less centrally concentrated. Possible consequences of the proposed mechanism for other problems in the CDM model and for the formation and early evolution of the baryonic component of galaxies are also briefly discussed.