2002ApJ...579..500T

We create stacked composite absorption spectra from Hubble Space Telescope Faint Object Spectrograph data from four quasi-stellar objects to search for absorption lines in the extreme-ultraviolet wavelength region associated with Lyα forest absorbers in the redshift range 1.6<z<2.9. We successfully detect O V λ630 in Lyα absorbers throughout the 10¹³-10^16.2 cm^–2 column density range. For a sample of absorbers with 10^13.2cm^–2<N(HI)<10^14.2cm^–2, corresponding to gas densities ranging from around the universal mean to overdensities of a few, we measure an O V λ630 equivalent width of 10.9±3.7 mÅ. We estimate that the detection is real with at least 99% confidence. We only detect O IV λ788, O IV λ554, O III λ833, and He I λ584 in absorbers with Lyα equivalent widths ≳0.6 Å, which are likely associated with traditional metal line systems. We find no evidence in any subsamples for absorption from N IV λ765, Ne V λ568, Ne VI λ559, Ne VIII λλ770, 780, or Mg X λλ610, 625. The measured equivalent widths of O V suggest values of OV/HI in the range -1.7 to -0.6 for 10^13.2cm^–2<N(HI)≲10¹⁵ cm^–2. The lack of detectable O IV absorption except in the strongest absorption systems suggests a hard ionizing background similar to the standard Haardt & Madau spectrum. Using photoionization models, we estimate that the oxygen abundance in the intergalactic medium with respect to the solar value is [O/H]~-2.2 to -1.3. Comparing to studies of C IV, we estimate [O/C]~0.3-1.2. The overabundance of oxygen relative to carbon agrees with other low-metallicity abundance measurements and suggests enrichment of the intergalactic medium by Type II supernovae.