The Detection of Oxygen in the Low-Density Intergalactic Medium

The abundances of metals in the intergalactic medium (IGM) can be used toconstrain the amount of star formation at high redshift and the spectral shapeof the ionizing background radiation. For both purposes it is essential tomeasure the abundances in regions of low density, away from local sources ofmetals and ionizing photons. Here we report the first detection of OVI in thelow-density IGM at high redshift. We perform a pixel-by-pixel search for OVIabsorption in eight high quality quasar spectra spanning the redshift range z =2.0-4.5. At 2 ~< z ~< 3, we clearly detect OVI in the form of a positivecorrelation between the HI Ly-alpha optical depth and the optical depth in thecorresponding OVI pixel, down to an HI optical depth of 0.1. This is an orderof magnitude lower in HI optical depth than the best CIV measurements can probeand constitutes the first clear detection of metals in underdense gas. Thenon-detection of OVI at z > 3 is consistent with the enhanced photoionizationfrom a hardening of the UV background below z ~ 3 but could also be caused bythe high level of contamination from Ly series lines.Comment: 5 pages, 2 figures. Reference added to match published versio