The Enrichment History of the Intergalactic Medium: Oviin Lyα Forest Systems at Redshiftz ∼ 2

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Abstract
A search for O VI 1031.93, 1037.62 Å at redshifts corresponding to Lyα lines in the zem ∼ 2.4 QSOs HE1122- 1648 and HE2217- 2818 reveals that a substantial fraction of those with H I column densities log N(HI) > 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 th e 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 ultr aviolet flux can reproduce the observations for densities that are in good agreement with a model assuming local, hydrostatic equilibrium and heavy element abundances in the range ∼ 10-3-10-2 solar. Photoionization by a UV flux much softer than that pred icted by Haardt & Madau (1996) 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 gravitation al confinement, unless the gas fraction is negligible. Subject headings: cosmology: miscellaneous — galaxies: formation — intergalactic medium — quasars: absorption lines — quasars: individual (HE1122 - 1648, HE2217- 2818)