[ITAL]Chandra[/ITAL] Detection of O [CSC]viii[/CSC] L[CLC]y[/CLC]α Absorption from an Overdense Region in the Intergalactic Medium

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 1.0 × 10 -5 cm -3 < n b < 7.5 × 10 -5 cm -3 (50 < δ b < 350) and a temperature of (4-5) × 10 6 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.