Dark Matter Equilibria in Galaxies and Galaxy Systems

Abstract

[abridged] In the dark matter (DM) halos embedding galaxies and galaxy systems the `entropy' K = σ^2 / ρ^{2/3} (a quantity that combines the radial velocity dispersion σwith the density ρ) is found from intensive N-body simulations to follow a powerlaw run K ~ r^α throughout the halos' bulk, with αaround 1.25. Taking up from phenomenology just that α~ 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 α-profiles, marked by the entropy slope αitself, and by the maximal gravitational pull κ_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 αto within the narrow range 1.25-1.29 from galaxies to galaxy systems. Our range of αapplies 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 α-profiles with parameters derived from the Jeans equation. We finally stress how our findings and predictions as to αand κ_crit contribute to understand hitherto unsolved issues concerning the fundamental structure of DM halos.

13 pages, 9 figures, uses RevTeX4 + emulateapj.cls and apjfonts.sty. Typos corrected. Accepted by ApJ