COLD DARK-MATTER COSMOLOGY WITH HYDRODYNAMICS AND GALAXY FORMATION - THE EVOLUTION OF THE INTERGALACTIC MEDIUM AND BACKGROUND-RADIATION FIELDS

We have supplemented our code, which computes the evolution of the physical state of a representative piece of the universe to include, not only the dynamics of dark matter (with a standard PM code), and the hydrodynamics of the gaseous component (including detailed collisional and radiative processes), but also galaxy formation on a heuristic but plausible basis. If, within a cell the ps is Jeans unstable, collapsing, and cooling rapidly, it is transformed to galaxy subunits, which are then followed with a collisionless code. These particles emit UV radiation and supernova blasts with energy efficiencies (in units of mc2) equal to (epsilon(UV), epsilon(SN)) = (10(-4), 10(-4.5)). This energy input significantly alters some aspects of the simulation; the primary consequences are to heat and ionize the ps at a much earlier epoch than if stellar feedback were ignored. We study two representative boxes with sizes L = (80, 8)h-1 Mpc, in both cases utilizing a 200(3) mesh containing 200(3) dark matter particles and having nominal resolutions of (400, 40)h-1 kpc, respectively, with true resolution approximately 2.5 times worse. We adopt the standard CDM perturbation spectrum with an amplitude Of sigma8 = (deltaM/M)rms,8 = 0.77, a compromise between the COBE normalization sigma8 = 1.05 and that indicated by the small-scale velocity dispersion (perhaps sigma8 = 0.45). We find that by the time that 0.2% of the baryons have been transformed to stars at redshift 8.6, reionization is 1/2 complete, and observed Gunn-Peterson limits to a redshift of z = 5 am satisfied. Very hot (10(7)-10(8) K) gas is produced by shocks in the clusters, lower temperature (10(6) K) ps from supernova-fed superwinds is in lower density filaments and sheets and photoheated ps (10(4)-10(5) K) fills the voids. The mass fractions in these components at redshift zero being roughly (29%, 40%, 31%) correspond to ps in the temperature ranges (< 10(5.5), 10(5.5)-10(6.5), > 10(6.5)) K. In rich clusters the galaxy and dark matter densities are more concentrated than the ps density in agreement with gravitational lens observations. We can combine the observed and computed baryon to total mass ratios with OMEGA(b) from light element nucleosynthesis, and we conclude that we live in an open universe with OMEGA(tot) = (0.11, 0.24) for h = (1.0, 0.5). While the correspondence between the thermodynamic properties of gas and radiation fields of this CDM model and the real world can be taken as support for the former, it is likely that many of the attributes of this simulation are generic, the result of a detailed physical treatment of atomic processes, and will be found as well in rival cosmological models.