  Adaptive SPH and N-body simulations were carried out to study the effect of
gasdynamics on the structure of dark matter halos that result from the
gravitational instability and fragmentation of cosmological pancakes. Such
halos resemble those formed in a hierarchically clustering CDM universe and
serve as a test-bed model for studying halo dynamics. With no gas, the density
profile is close to the universal profile identified previously from N-body
simulations of structure formation in CDM. When gas is included, the gas in the
halo is approximately isothermal, and both the dark matter and the gas have
singular central density profiles which are steeper than that of the dark
matter with no gas. This worsens the disagreement between observations of
constant density cores in cosmological halos and the singular ones found in
simulations. We also find that the dark matter velocity distribution is less
isotropic than found by N-body simulations of CDM, because of the strongly
filamentary substructure.
