Far-UV echelle spectroscopy of the radio-quiet QSO H1821+643 ( z _ { em } = 0.297 ) , obtained with the Space Telescope Imaging Spectrograph ( STIS ) at \sim 7 km s ^ { -1 } resolution , reveals 4 definite O vi absorption line systems and one probable O vi absorber at 0.15 < z _ { abs } < 0.27 .
The four definite O vi absorbers are located near galaxies and are highly displaced from the quasar in redshift ; these are likely intervening systems unrelated to the background QSO .
In the case of the strong O vi system at z _ { abs } = 0.22497 , multiple components are detected in Si iii and O vi as well as H i Lyman series lines , and the differing component velocity centroids and b - values firmly establish that this is a multiphase absorption system .
A weak O vi absorber is detected at z _ { abs } = 0.22637 , i.e. , offset by \sim 340 km s ^ { -1 } from the z _ { abs } = 0.22497 system .
Ly \alpha absorption is detected at z _ { abs } = 0.22613 , but no Ly \alpha absorption is significantly detected at 0.22637 .
Other weak O vi absorbers at z _ { abs } = 0.24531 and 0.26659 and the probable O vi system at 0.21326 have widely diverse O vi/H i column density ratios with N ( O vi ) / N ( H i ) ranging from \leq 0.14 \pm 0.03 to 5.2 \pm 1.2 .
The number density of O vi absorbers with rest equivalent width > 30 mÅ in the H1821+643 spectrum is remarkably high , dN / dz \sim 48 , with a high ( 90 % ) confidence that it is greater than 17 .
We conservatively estimate that the cosmological mass density of the O vi systems \Omega _ { b } ( { O VI } ) \gtrsim 0.0008 h _ { 75 } ^ { -1 } .
With an assumed metallicity of 1/10 solar and a conservative assumption that the fraction of oxygen in the O vi ionization stage is 0.2 , we obtain \Omega _ { b } ( { O VI } ) \gtrsim 0.004 h _ { 75 } ^ { -1 } .
This is comparable to the combined cosmological mass density of stars and cool gas in galaxies and X-ray emitting gas in galaxy clusters at low redshift .