  An analytical model for the postcollapse equilibrium structure of virialized
objects which condense out of the cosmological background universe is described
and compared with observations and simulations of cosmological halos. The model
is based upon the assumption that virialized halos are isothermal, which leads
to a prediction of a unique nonsingular isothermal sphere for the equilibrium
structure, with a core density which is proportional to the mean background
density at the epoch of collapse. These predicted nonsingular isothermal
spheres are in good agreement with observations of the internal structure of
dark-matter-dominated halos from dwarf galaxies to X-ray clusters. Our model
also reproduces many of the average properties of halos in CDM simulations to
good accuracy, suggesting that it is a useful analytical approximation for
halos which form from realistic initial conditions. While CDM N-body
simulations find profiles with a central cusp, our nonsingular model matches
the simulated halos outside the innermost region well. This model may also be
of interest as a description of halos in nonstandard CDM models like
self-interacting dark matter, which have been proposed to eliminate the
discrepancy between the cuspy halos of standard CDM simulations and observed
halos with uniform-density cores.
