On the structure of dark matter halos at the damping scale of the power spectrum with and without relict velocities

We report a series of high-resolution cosmological N-body simulations designed to explore the formation and properties of dark matter halos with masses close to the damping scale of the primordial power spectrum of density fluctuations. We further investigate the effect that the addition of a random component, v(rms), into the particle velocity field has on the structure of halos. We adopted as a fiducial model a Lambda WDM cosmology with a nonthermal sterile neutrino mass of 0.5 keV. The filtering mass corresponds then to M-f = 2.6 x 10(12) h(-1) M-circle dot. Halos of masses close to M-f were simulated with several million particles. The results show that, on the one hand, the inner density slope of these halos (at radii less than or similar to 0.02 the virial radius R-v) is systematically steeper than the one corresponding to the NFW fit or to the CDM counterpart. On the other hand, the overall density profile (radii > 0.02R(v)) is less curved and less concentrated than the NFW fit, with an outer slope shallower than -3. For simulations with vrms, the inner halo density profiles flatten significantly at radii smaller than 2-3 h(-1) kpc (less than or similar to 0.010R(v)-0.015R(v)). A constant density core is not detected in our simulations, with the exception of one halo for which the flat core radius is approximate to 1 h(-1) kpc. Nevertheless, if cored'' density profiles are used to fit the halo profiles, the inferred core radii are approximate to(0.1-0.8) h(-1) kpc, in rough agreement with theoretical predictions based on phase-space constraints and on dynamical models of warm gravitational collapse. A reduction of v(rms) by a factor of 3 produces a modest decrease in core radii, by less than a factor of 1.5. We discuss the extension of our results into several contexts, for example, to the structure of the cold DM microhalos at the damping scale of this model.