Cooling pancakes

Abstract

In theories of galaxy formation with a damping cut-off in the density fluctuation spectrum, the first non-linear structures to form are Zeldovich pancakes in which dissipation separates gas from any collisionless dark matter then present. One-dimensional numerical simulations of the collapse, shock heating, and subsequent thermal evolution of pancakes are described. Neutrinos (or any other cool collisionless particles) are followed by direct N-body methods and the gas by Eulerian hydrodynamics with conduction as well as cooling included. We find the pressure is relatively uniform within the shocked region and approximately equals the instantaneous ram pressure acting at the shock front. An analytic theory based upon this result accurately describes the numerical calculations. The fraction of baryons which cool below a given temperature depends sensitively upon the total mass of the collapsing system, the baryon density parameter Ω_B and somewhat upon the inclusion of conduction at high mass. For example, if the co-moving wavelength, L, exceeds 25 Mpc and $$\Omega_\text B \lt 0.1$$, less than 15 per cent of the gas cools below 10⁴ K by z = 3 if collapse occurs at redshift 5; if $$L \gt 50$$ Mpc, cool layers which do form eventually disappear due to an inward-moving conduction front. By utilizing anisotropic expansion factors, we simulate transverse flow toward string-like structures. In some cases, the increased baryon density can overcome the effects of adiabatic compression to enhance cooling. Cooling is also efficient if the dark matter is in the form of particles which interact more weakly than neutrinos, since then the collapsing mass is smaller.