THE UNIVERSE IN A BOX - THERMAL EFFECTS IN THE STANDARD COLD DARK MATTER SCENARIO

We compute the evolution of a representative piece of the universe on the assumption that the cold dark matter scenario is correct. A standard particle-mesh dark matter code is coupled with a flux-based mesh code originally designed for engineering hydrodynamical applications. A variety of checks have been made which indicate that the resolution of the code is a few cells, giving accuracy for integral quantities in our simulations of 1%-3%. In a first application of the new code to a cube of 30h-1 Mpc containing 106 cells, we find the following results: 1. On scales greater than 1h-1 Mpc, bias is inverse in the sense that dark matter is more clumped than gas. 2. Bremsstrahlung and Compton cooling are unimportant on scales ≥1 Mpc; shocks gradually heat the gas, leaving most of it in voids at T ≤103.5 K but with a small fraction reaching T ≥107 K. 3. Ultraviolet and soft X-ray emission from the heated gas is significant, providing an important part of the background radiation field Jv and significant ionizing of the IGM. 4. The mean induced (ΔT/T) for microwave background radiation in the Rayleigh-Jeans part of the spec-trum (due to Sunyaev-Zel'dovich effect) is 1.1×10-6 with fluctuations of 5.6×10-7 on arcminute scales and a mean y parameter of 5.5×10-7, which is potentially detectable by the COBE satellite. Results (3) and (4) quoted above are dependent on normalization, which was chosen to be (ΔM/M)rms = 0.5 on a top hat 8h-1 Mpc scale (bias = 2). Increasing the amplitude (reducing bias b) would increase the computed levels of y by b-4.6 and of J by b-4.3.