The cosmological density and ionization of hot gas: O VI absorption in quasar spectra

We have conducted the first survey for O vr 1032, 1038 Angstrom absorption lines in QSO spectra. We used medium-resolution (R approximate to 1300) high signal-to-noise (approximate to 20) Faint Object Spectrograph spectra of 11 QSOs (0.53 less than or equal to z(em) less than or equal to 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 Ly alpha 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 Angstrom. 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 11 absorbers. In 7 of the 12 O VI systems, O vr, LyB 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 LyB 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 vr lines are on the linear part of the curve of growth, we estimate the lower limit of the cosmological mass density, Omega(O vI)greater than or equal to 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.. greater than or equal to -2.4 and much greater if O vr is not the dominant ion of oxygen.