Dark Matter Equilibria in Galaxies and Galaxy Systems
  [abridged] In the dark matter (DM) halos embedding galaxies and galaxy systems the `entropy' K = \sigma^2 / \rho^{2/3} (a quantity that combines the radial velocity dispersion \sigma with the density \rho) is found from intensive N-body simulations to follow a powerlaw run K ~ r^{\alpha} throughout the halos' bulk, with \alpha around 1.25. Taking up from phenomenology just that \alpha ~ const applies, we cut through the rich analytic contents of the Jeans equation describing the self-gravitating equilibria of the DM; we specifically focus on computing and discussing a set of novel physical solutions that we name \alpha-profiles, marked by the entropy slope \alpha itself, and by the maximal gravitational pull \kappa_crit required for a viable equilibrium to hold. We then use an advanced semianalytic description for the cosmological buildup of halos to constrain the values of \alpha to within the narrow range 1.25-1.29 from galaxies to galaxy systems. Our range of \alpha applies since the transition time that - both in oursemianalytic description and in state-of-the-art numerical simulations - separates two development stages: an early violent collapse that comprises a few major mergers and enforces dynamical mixing, followed by smoother mass addition through slow accretion. We also give an accurate analytic representation of the \alpha-profiles with parameters derived from the Jeans equation. We finally stress how our findings and predictions as to \alpha and \kappa_crit contribute to understand hitherto unsolved issues concerning the fundamental structure of DM halos. 
A. Lapi (1,2), A. Cavaliere (1,3) ((1)Univ. "Tor Vergata", Roma,   Italy; (2) SISSA/ISAS, Trieste, Italy; (3) Accademia Linc...