Chandra detection of O VIII Lyα absorption from an overdense region in the intergalactic medium

We report the first detection of an O vIII Lyalpha 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 sigma level with cz approximate to 16,600 km s(-1). At the same velocity, Shull et al. detected a small group of spiral galaxies (with an overdensity of delta(gal) similar to100) and low-metallicity H I Lyalpha clouds. We constrain the intragroup gas that gives rise to the O vIII Lyalpha line to a baryon density in the range 1.0 x 10(-5) cm(-3) < n(b) < 7.5 x 10(-5) cm(-3) (50 < delta(b) < 350) and a temperature of (4-5) x 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" (similar to10%, or similar to30%-40% of the WHIM) are probed by the O vIII absorber.