Lyα 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 (IGM) along the sightline towards PKS 2155-304 with the Chandra Low Energy Transmission Grating Spectrometer (LETGS). The absorption line is detected at 4.5σ level with cz ≈ 16, 600 km s. At the same velocity Shull et al. (1998) detected a small group of spiral galaxies (with an overdensity of δgal ∼ 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 < nb < 7.5 × 10 cm (50 < δb < 350) and a temperature of 4 − 5 × 10 K, assuming 0.1 solar abundance. 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 a large fraction of the “missing baryons” (∼ 10%, or ∼ 30-40% of the WHIM) are probed by the O VIII absorber. Subject headings: intergalactic medium — quasars: absorption lines — largescale structure of universe — BL Lacertae objects: individual (PKS 2155-304)