  In order to study their interplay with large scale cosmic evolution and with
relativistic effects, such as gravitational lenses, quintessence sources or
gravitational waves, we construct a post-Newtonian fluid framework for the
"Navarro-Frenk-White'' (NFW) models of galactic halos that follow from N-body
numerical simulations. Since these simulations are unable to resolve regions
very near the halo center, the extrapolation of the fitting formula leads to a
spherically averaged "universal'' density profile that diverges at the origin.
We remove this inconvenient feature by replacing a small central region of the
NFW halo with an interior Schwarzschild solution with constant density,
continuously matched to the remaining NFW spacetime. A model of a single halo,
as an isolated object with finite mass, follows by smoothly matching the NFW
spacetime to a Schwarzschild vacuum exterior along the virial radius, the
physical "cut-off'' customarily imposed, as the mass associated with NFW
profiles diverges asymptotically. Numerical simulations assume weakly
interacting collisionless particles, hence we suggest that NFW halos
approximately satisfy an "ideal gas'' type of equation of state, where
mass-density is the dominant rest--mass contribution to matter--energy, with
the internal energy contribution associated with an anisotropic kinetic
pressure. We show that, outside the central core, this pressure and the mass
density roughly satisfy a polytropic relation. Since stellar polytropes are the
equilibrium configurations in Tsallis' non-extensive formalism of Statistical
Mechanics, we argue that NFW halos might provide a rough empirical estimate of
the free parameter $q$ of Tsallis' formalism.
