2002ApJ...572L.127F

We report the first detection of an O VIII Lyα absorption line associated with an overdense region in the intergalactic medium along the sight line toward PKS 2155-304 with the Chandra Low Energy Transmission Grating Spectrometer. The absorption line is detected at the 4.5 σ level with cz~16,600 km.s^–1. At the same velocity, Shull et al. detected a small group of spiral galaxies (with an overdensity of δ_gal∼100) and low-metallicity H I Lyα clouds. We constrain the intragroup gas that gives rise to the O VIII Lyα line to a baryon density in the range 1.0x10^–5 cm^–3<n_b<7.5x10^–5 cm^–3 (50<δ_b<350) and a temperature of (4-5)x10⁶ K, assuming 0.1 Z_☉. These estimates are in accordance with those of the warm/hot intergalactic medium (WHIM) that are predicted from hydrodynamic simulations. Extrapolating from this single detection implies that a large fraction of the ``missing baryons'' (∼10%, or ∼30%-40% of the WHIM) are probed by the O VIII absorber.