A detailed view of filaments and sheets in the warm-hot intergalactic medium

Context. Numerical simulations predict that a considerable fraction of the missing baryons at redshift z ≈ 0 rest in the so-called warm-hot intergalactic medium (WHIM). The filaments and sheets of the WHIM have high temperatures (105 − 107 K) and a high degree of ionization but only low to intermediate densities. Therefore, their reliable detection is a challenging task for today’s observational cosmology. The particular physical conditions of the WHIM structures, e.g. density and temperature profiles, or velocity fields, are expected to leave their special imprint on spectroscopic observations. Aims. In order to get further insight into these conditions, we performed hydrodynamical simulations of the WHIM. Instead of analyzing extensive simulations of cosmological structure formation, we simulate certain well-defined structures and studied the impact of different physical processes as well as of the scale dependencies. Methods. We started with a comprehensive study of the one-dimensional collapse (pancake) and examined the influence of radiative cooling, heating due to an UV background, and thermal conduction. We investigated the effect of small-scale perturbations given by the cosmological power spectrum. Results. 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 longer 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 longer than L ≈ 30 Mpc. Though the physics in the corresponding three-dimensional case is much more complex, one might expect a similar regulation mechanism with respect to the cold streams along filaments, too. However, this question will be addressed in a forthcoming paper. The obtained detailed profiles for density and temperature for prototype WHIM structures allow for the determination of possible spectral signatures by the WHIM