A search for O VI 1031.93, 1037.62 ? at redshifts corresponding to Lyα lines in the zem ~ 2.4 QSOs HE 1122-1648 and HE 2217-2818 reveals that a substantial fraction of those with H I column densities log N(H I) 14.0 (cm-2) are highly ionized and show some heavy-element enrichment. If these two sight lines are typical, then the O VI systems contain a cosmologically significant fraction of the baryons and the metals in the universe. For most systems the temperatures derived from the line widths are too low for collisional ionization to be responsible for the O VI lines. Photoionization models with a substantial hard UV flux can reproduce observations for densities that are in good agreement with a model assuming local, hydrostatic equilibrium and heavy-element abundances in the range ~10-3 to 10-2 solar. Photoionization by a UV flux much softer than that predicted by Haardt & Madau for a background dominated by quasars can be ruled out. Finally, we find one system with a very low H I column density for which both photoionization and collisional ionization models yield a metallicity close to solar and a density that is inconsistent with gravitational confinement, unless the gas fraction is negligible.
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