The low-z intergalactic medium.: I.: OVI baryon census

Intergalactic absorbers along lines of sight to distant quasars are a powerful diagnostic for the evolution and content of the intergalactic medium (IGM). In this study, we use the Far Ultraviolet Spectroscopic Explorer ( FUSE) satellite to search 129 known Ly alpha absorption systems at z < 0.15 toward 31 active galactic nuclei (AGNs) for corresponding absorption from higher Lyman lines and the important metal ions O VI and C III. We detect O VI in 40 systems, over a smaller range of column density (log N-O (VI) = 13.0-14.35) than seen in H I (log N-H (I) = 13.0-16.0). The coexistence of O VI and H I suggests a multiphase IGM with warm photoionized and hot ionized components. With improved O VI detection statistics, we find a steep distribution in O VI column density, dN(O) (VI)/dN(O) (VI) proportional to NO (-2.2 +/- 0.1)(VI) O vi, suggesting that numerous weak O vi absorbers contain baryonic mass comparable to the rare strong absorbers. Down to 30 m angstrom equivalent width (O VI lambda 1032) we find an absorber frequency dN(O) (VI)/dz approximate to 17 +/- 3. The total cosmological mass fraction in this hot gas is at least Omega(WHIM) (0.0022 +/- 0.0003)[h(70)(Z(O)/0.1 Z(circle dot))(f(O) (VI)/0.2)](-1), where we have scaled to fiducial values of oxygen metallicity, O VI ionization fraction, and the Hubble constant. Gas in the warm-hot intergalactic medium (WHIM) at 10(5)-10(6) K contributes at least 4.8% +/- 0.9% of the total baryonic mass at z < 0.15. We then combine empirical scaling relations for the observed "multiphase ratio,'' N-H (I)/N-O (VI) proportional to N-H (0.9 +/- 0.1)(I) and for hydrogen overdensity in cosmological simulations, N-H (I) proportional to delta(0.7)(H) with the H I photoionization correction to derive the mean oxygen metallicity, Z(O) approximate to (0.09 Z(circle dot))(f(O) (VI)/0.2)(-1) in the low-z multiphase gas. Given the spread in the empirical relations and in f(O) (VI), the baryon content in the O VI WHIM could be as large as 10%. Our survey is based on a large improvement in the number of O VI absorbers (40 vs. 10) and total redshift path length (Delta z approximate to 2.2 vs. Delta z approximate to 0.5) compared to earlier surveys.