We report the first detection of an O viii Ly \alpha 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 4.5 \sigma level with cz \approx 16 , 600 ~ { } km s ^ { -1 } .
At the same velocity Shull et al .
( 31 ) detected a small group of spiral galaxies ( with an overdensity of \delta _ { gal } \sim 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 } < n _ { b } < 7.5 \times 10 ^ { -5 } ~ { } cm ^ { -3 } ( 50 < \delta _ { b } < 350 ) and a temperature of 4 - 5 \times 10 ^ { 6 } 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 ” ( \sim 10 % , or \sim 30-40 % of the WHIM ) are probed by the O viii absorber .