Nonequilibrium thermodynamics and cosmological pancake formation

We investigate the influence of nonequilibrium thermodynamics on cosmologic al structure formation. In this paper, we consider the collapse of the plan ar perturbations usually called "Zeldovich pancakes." We have developed for that purpose a new two-fluid (gas and dark matter) hydrodynamical code, wi th three different thermodynamical species: electrons, ions, and neutral pa rticles, T-e not equal T-l not equal T-n. We describe in detail the complex structure of accretion shock waves. We include several relevant processes for a low-density, high-temperature collisional plasma: nonequilibrium chem ical reactions, cooling, shock heating, thermal energy equipartition betwee n electrons, ions, and neutral particles, and electronic conduction. We fin d two different regions in the pancake structure: a thermal precursor ahead of the compression front and an equipartition wave after the compression f ront, where electron and ion temperatures differ significantly. This comple x structure may have two interesting consequences: preheating of unshocked regions in the vicinity of massive X-ray clusters and ion and electron temp erature differences in the outer regions of X-ray clusters.