The Cosmological Density and Ionization of Hot Gas: O VI Absorption in Quasar Spectra

Abstract

We have conducted the first survey for O VI 1032, 1038 A absorption lines in QSO spectra. We used medium-resolution (R ~ 1300) high signal- to-noise (~20) Faint Object Spectrograph spectra of 11 QSOs (0.53 <= z_em_ <= 2.07) from the Hubble Space Telescope archive. We use simulated spectra to determine the significance of the line identifications, which lie exclusively in the Lyα forest. We found 12 O VI doublets, of which nine are expected to be real and six constitute a uniform sample with both lines exceeding a rest equivalent width of W_r_ = 0.21 A. The number of O VI doublets per unit redshift at a mean absorption redshift of z_ave_ = 0.9 is <N(z)> = 1.0 +/- 0.6, which is similar to the density of C IV and Mg II absorbers. In 7 of the 12 O VI systems, O VI, Lyβ and C IV lines have similar equivalent widths and are probably photoionized. In each of the remaining five systems, O VI has larger equivalent widths than those detected for Lyβ and C IV. These systems are labeled as high ionization and are likely to be because of collisional ionization. These would be the first QSO absorption systems known to be collisionally ionized. Assuming that the O VI lines are on the linear part of the curve of growth, we estimate the lower limit of the cosmological mass density, {OMEGA}(O VI)>= 7 x 10^-8^ h_100_^-1^. Since O > O VI, if the mean cosmic metallicity Z were below 4 x 10^-3^ solar, then the accompanying hydrogen and helium would account for all baryons in the universe. We conclude that log Z(z = 0.9)/Z_sun_ >= - 2.4 and much greater if O VI is not the dominant ion of oxygen.