A detailed view of filaments and sheets in the warm-hot intergalactic
  medium. I. Pancake formation
  Numerical simulations predict a considerable fraction of the missing baryons
at redshift z ~ 0 resting in the so called warm-hot intergalactic medium
(WHIM). The filaments and sheets of the WHIM have high temperatures 10^5 - 10^7
K) and a high degree of ionization while having only low to intermediate
densities. The particular physical conditions of the WHIM structures, e.g.
density and temperature profiles, velocity fields, are expected to leave their
special imprint on spectroscopic observations. In order to get further insight
into these conditions, we perform hydrodynamical simulations of the WHIM.
Instead of analyzing large simulations of cosmological structure formation, we
simulate particular well-defined structures and study the impact of different
physical processes as well as of the scale dependencies. We start with the
comprehensive study of the one-dimensional collapse (pancake) and examine the
influence of radiative cooling, heating due to an UV background, and thermal
conduction. We investigate the effect of small scale perturbations given
according to the initial cosmological power spectrum. If the initial
perturbation length scale L exceeds ~ 2 Mpc the collapse leads to shock
confined structures. As a result of radiative cooling and of heating due to an
UV background a relatively cold and dense core forms in the one-dimensional
case. The properties of the core (extension, density, and temperature) are
correlated with L. For larger L the core sizes are more concentrated. Thermal
conduction enhances this trend and may even result in an evaporation of the
core. Our estimates predict that a core may start to evaporate for perturbation
lengths larger than L ~ 30 Mpc. The obtained detailed profiles for density and
temperature for prototype WHIM structures allow for the determination of
possible spectral signatures by the WHIM.
