Formation and structure of halos in a warm dark matter cosmology

Using high-resolution cosmological N-body simulations, we study how the den sity profiles of dark matter halos are affected by the filtering of the den sity power spectrum below a given scale length and by the introduction of a thermal velocity dispersion. In the warm dark matter (WDM) scenario, both the free-streaming scale, R-f, and the velocity dispersion, v(th)(w), are d etermined by the mass, m(W), of the WDM particle. We found that v(th)(w) is too small to affect the density profiles of WDM halos. Down to the resolut ion attained in our simulations (similar to0.01 virial radii), there is not any significant difference in the density profiles and concentrations of h alos obtained in simulations with and without the inclusion of v(th)(w). Re solved soft cores appear only when we artificially increase the thermal vel ocity dispersion to a value that is much higher than v(th)(w). We show that the size of soft cores in a monolithic collapse is related to the tangenti al velocity dispersion. The density profiles of the studied halos with mass es down to similar to0.01 the filtering mass M-f can be described by the Na varro-Frenk-White shape; soft cores are not formed. Nevertheless, the conce ntrations of these halos are lower than those of the CDM counterparts and a re approximately independent of mass. The cosmogony of halos with masses le ss than or similar to M-f is not hierarchical: they form through monolithic collapse and by fragmentation of larger structures. The formation epoch of these halos is slightly later than that of halos with masses approximate t o M-f. The lower concentrations of WDM halos with respect to their CDM coun terparts can be accounted for by their late formation epoch. Overall, our r esults point to a series of advantages of a WDM model over the CDM one. In addition to solving the substructure problem, a WDM model with R-f similar to 0.16 Mpc (m(W) approximate to 0.75 keV; flat cosmology with Omega (Lambd a) = h = 0.7) also predicts concentrations, a Tully-Fisher relation, and fo rmation epochs for small halos, which seems to be in better agreement with observations than CDM predictions.