Citation:
Date Published:
junAbstract:
{We report the first detection of an O VIII Ly{$\alpha$} 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\~{}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 Ly{$\alpha$} clouds. We constrain the intragroup gas that gives rise to the O VIII Ly{$\alpha$} line to a baryon density in the range 1.0{\times}10$^{-5}$ cm$^{-3}$łt}n$_{b}$łt}7.5{\times}10$^{-5}$ cm$^{-3}$ (50łt}{$δ$}$_{b}$łt}350) and a temperature of (4-5){\times}10⁶ K, assuming 0.1 Z$_{solar}$. 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. }