About half of the baryons in the local Universe are not yet observed and are thought to reside in the intergalactic medium at temperatures of 0.1-10 million K and densities of 10-6 -10-4 cm-3 . This is called warm-hot intergalactic medium (WHIM). In this paper, we summarize search for emission from the WHIM, and show prospects with future instruments. Using XIS instrument onboard Suzaku, we searched for redshifted OVII and OVIII WHIM emission lines, in cluster outskirts and superclusters: Shapley supercluster, Sculptor supercluster, A2218, A1413, A2142, A2052, and the Coma cluster. The chance for detecting a signal was estimated to be sufficient to motivate our search, given the relatively high density and high temperature expected for the WHIM in these regions. We did not detect the significant emission, but obtained upper limits in OVII and OVIII emission intensities for these clusters and superclusters. The density of the WHIM was constrained based on the upper limits, which is 200-300 times mean baryon density of the Universe. Future instruments such as microcalorimeters have much higher sensitivity for weak lines. In particular a combination of a microcalorimeter and large grasp (effective area times field of view) telescope is ideal for mapping of the WHIM. We created mock spectra that contain the WHIM, cosmic X-ray background and Galactic emission. Here we present our results concerning detectability and expected 3-d maps of the line emitting regions asssociated to the WHIM, with future missions such as EDGE/XENIA.
Takei, Y., Ohashi, T., Sato, K., Mitsuishi, I., Branchini, E.F., Ursino, E., et al. (2010). Search for emission from warm-hot intergalactic medium. In 3Proceedings of the 8th COSPAR Scientific Assembly.. Bremen : 38th COSPAR Meeting.