Detectability of the Warm/Hot Intergalactic Medium through Emission Lines of O VII and O VIII

Most of the cosmic baryons predicted by the big-bang nucleosynthesis have evaded direct detection. Recent numerical simulations indicate that approximately 30 to 50 percent of the total baryons in the present universe are supposed to take the form of warm/hot intergalactic medium (WHIM) whose X-ray continuum emission is very weak. To identify those missing baryons, we consider in detail the detectability of WHIM directly through the emission lines of O VII (561, 568, 574, 665 eV) and O VIII (653 eV). For this purpose, we create mock spectra of the emission lines of WHIM using a light-cone output of cosmological hydrodynamic simulations. While the predicted fluxes are generally below the current detection limit, an unambiguous detection will be feasible with a dedicated X-ray satellite mission, which we also discuss in detail. Our proposed mission is especially sensitive to a WHIM with gas temperature of $T = 10^{6 \hbox{--} 7} \,\mathrm{K}$ and overdensity of $\delta = 10 \hbox{--} 100$ up to a redshift of 0.3 without being significantly contaminated by the cosmic X-ray background and the Galactic emissions. Thus, such a mission provides a unique and important tool to identify a large fraction of otherwise elusive baryons in the universe.