The detection of oxygen in the low-density intergalactic medium

The abundances of metals in the intergalactic medium (IGM) can be used to constrain the amount of star formation at high redshift and the spectral shape of the ionizing background radiation. For both purposes, it is essential to measure the abundances in regions of low density, away from local sources of metals and ionizing photons. Here we report the first detection of O VI in the low-density IGM at high redshift. We perform a pixel-by-pixel search fur O VI absorption in eight high-quality quasar spectra spanning the redshift range z = 2.0-4.5. At 2 less than or similar to z less than or similar to 3, we clearly detect O VI in the form of a positive correlation between the H I Ly alpha optical depth and the optical depth in the corresponding O VI pixel, down to tau(HI) similar to 10(-1). This is an order of magnitude lower in tau(HI) than the best C IV measurements can probe, and it constitutes the first clear detection of metals in slncEerdense gas. The nondetection of O VI at z > 3 is consistent with the enhanced photoionization from a hardening of the UV background below z similar to 3 but could also be caused by the high level of contamination from Lyman series lines.