Where are the baryons? III. Nonequilibrium effects and observables

A significant fraction (40% - 50%) of baryons at the present epoch are predicted to be shock-heated to the warm-hot intergalactic medium ( WHIM) by our previous numerical simulations. Here we recompute the evolution of the WHIM with several major improvements: ( 1) galactic superwind feedback processes from galaxy and star formation are explicitly included; ( 2) major metal species (O V to O IX) are computed explicitly in a nonequilibrium way; and ( 3) mass and spatial dynamic ranges are larger by factors of 8 and 2, respectively, than in our previous simulations. We find the following: ( 1) Nonequilibrium calculations produce significantly different results than do ionization equilibrium calculations. ( 2) The abundance of O VI absorption lines based on nonequilibrium simulations with galactic superwinds is in remarkably good agreement with the latest observations, strongly validating our model, while the predicted abundances for O VII and O VIII absorption lines appear to be lower than the still very uncertain observations. The expected abundances for O VI ( as well as Ly alpha), O VII, and O VIII absorption systems are in the range 50 - 100 per unit redshift at equivalent width EW = 1 km s(-1), decreasing to 10 - 20 per unit redshift at EW = 10 km s(-1), to one to three lines for O VII and O VIII and negligible for O vi at EW > 100 km s(-1). ( 3) Emission lines, primarily O VI and Ly alpha in the UV and O VII and O VIII in soft X-rays, are potentially observable by future missions, and different lines provide complementary probes of the WHIM in the temperature-density-metallicity phase space. The number of emission lines per unit redshift that may be detectable by planned UV and soft X-ray missions are of order 0.1-1.