Where Are the Baryons? III. Nonequilibrium Effects and Observables

Numerical simulations of the intergalactic medium have shown that at thepresent epoch a significant fraction (40-50%) of the baryonic component shouldbe found in the (T~10^6K) Warm-Hot Intergalactic Medium (WHIM) - with severalrecent observational lines of evidence indicating the validity of theprediction. We here recompute the evolution of the WHIM with the followingmajor improvements: (1) galactic superwind feedback processes from galaxy/starformation are explicitly included; (2) major metal species (O V to O IX) arecomputed explicitly in a non-equilibrium way; (3) mass and spatial dynamicranges are larger by a factor of 8 and 2, respectively, than in our previoussimulations. We find: (1) non-equilibrium calculations produce significantlydifferent results from ionization equilibrium calculations. (2) The abundanceof O VI absorption lines based on non-equilibrium simulations with galacticsuperwinds is in remarkably good agreement with latest observations, implyingthe validity of our model, while the predicted abundances for O VII and O VIIIabsorption lines appear to be lower than observed but the observationalerrorbars are currently very large. The expected abundances for O VI (as wellas Lyman alpha), O VII and O VIII absorption systems are in the range 50-100per unit redshift at EW=1km/s decreasing to 10-20 per unit redshift atEW=10km/s. The number of O VI absorption lines with EW>100km/s is very small,while there are about 1-3 lines per unit redshift for O VII and O VIIIabsorption lines at EW=100km/s. (3) Emission lines, primarily O VI and \lya inthe UV and O VII and O VIII in the soft X-rays are potentially observable byfuture missions. The number of emission lines per unit redshift that may bedetectable by planned UV and soft X-ray missions are in the order of 0.1-1.