Structural properties of Dark Matter Halos

Using the results of a high mass resolution ($256^{3}$ particles) N-body simulation of a cluster-forming region, we study the statistic $\sigma_{v}-M$ for low mass halos (${\rm M} < 10^{12} {\rm M}_{\sun}$), and we compare it with three theoretical models. At the final redshift halos are well described by the Truncated Isothermal Sphere model recently introduced by Shapiro, Iliev & Raga (1999). We speculate that this is a consequence of the action of tidal fields on galaxy formation within clusters.