  Adaptive SPH and N-body simulations were carried out to study the collapse
and evolution of dark matter halos that result from the gravitational
instability and fragmentation of cosmological pancakes. Such halos resemble
those formed by hierarchical clustering in a CDM universe and serve as a
convenient test-bed model for studying halo dynamics. Our halos are in
approximate virial equilibrium and roughly isothermal, as in CDM simulations.
Their density profiles agree quite well with the fit to N-body results for CDM
halos by Navarro, Frenk, & White (NFW). This test-bed model enables us to study
the evolution of individual halos. The masses of our halos evolve in three
stages: an initial collapse, continuous infall, and a final stage in which
infall tapers off as a result of finite mass supply. In the continuous infall
stage, halo mass grows at the rate expected for self-similar spherical infall,
with M(a) proportional to the scale factor a. After the end of initial collapse
at a=a_0, the concentration parameter grows linearly with a, c(a)~4a/a_0. The
virial ratio 2T/|W| just after virialization is about 1.35, as predicted by the
truncated isothermal sphere model and consistent with the value expected for a
virialized halo in which mass infall contributes an effective surface pressure.
Thereafter, the virial ratio evolves towards the value expected for an isolated
halo, 2T/|W|~1. This mass accretion history and evolution of concentration
parameter are very similar to those reported recently in N-body simulations of
CDM. We therefore conclude that the fundamental properties of halo formation
and evolution are generic to the formation of cosmological halos by
gravitational instability and are not limited to hierarchical collapse
scenarios or even to Gaussian-random-noise initial conditions.
