Searching for cosmic missing baryons with DIOS - Diffuse intergalactic oxygen surveyor

Approximately 30 to 50 percent of the total baryons in the present universe is supposed to take the form of wa arm/hot intergalactic medium (WHIM) whose X-ray continuum emission is very weak. In order to carry out a direct and homogeneous survey of elusive cosmic missing baryons, we propose a dedicated soft-X-ray mission, DIOS (Diffuse Intergalactic Oxygen Surveyor). The unprecedented energy resolution (∼ 2eV) of the XSA (X-ray Spectrometer Array) on-board DIOS enables us to identify WHIM with gas temperatures T = 10^6-7K and overdensity δ= 10-100 located at z < 0.3 through emission lines of OVII and OVIII. DIOS, which will hopefully be launched in several years time, promises to open a new window of detection and characterization of cosmic missing baryons and to provide yet another important and complementary tool to trace the large-scale structure of the universe.