Intervening O VI Quasar Absorption Systems at Low Redshift: A Significant Baryon Reservoir

Far-UV echelle spectroscopy of the radio-quiet QSO H1821+643 (z_em=0.297), obtained with the Space Telescope Imaging Spectrograph (STIS) at ~7 km s^-1 resolution, reveals four 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 ~340 km s^-1 from the z_abs=0.22497 system. Lyα absorption is detected at z_abs=0.22613, but no Lyα 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 <=0.14+/-0.03 to 5.2+/-1.2. The number density of O VI absorbers with rest equivalent width greater than 30 mÅ in the H1821+643 spectrum is remarkably high, dN/dz~48, which implies with a high (90%) confidence that it is greater than 17 in the low-redshift intergalactic medium. We conservatively estimate that the cosmological mass density of the O VI systems is Ω_b(OVI)>~0.0008 h^-1₇₅. 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 Ω_b(OVI)>~0.004 h^-1₇₅. 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. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. The data were reduced with the STIS Team software, and the research was supported by NASA through grants GO-08165.01-97A and GO-08165.02-97A from STScI.