Extreme-ultraviolet absorption lines in Lyα forest absorbers and the oxygen abundance in the intergalactic medium

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 Lyalpha forest absorbers in the redshift range 1.6<2.9. We successfully detect O v lambda630 in Lyalpha absorbers throughout the 10(13)-10(16.2) cm(-2) column density range. For a sample of absorbers with 10(13.2) cm(-2) < N(H I)<10(14.2) cm(-2), corresponding to gas densities ranging from around the universal mean to overdensities of a few, we measure an O v lambda630 equivalent width of 10.9+/-3.7 mAngstrom. We estimate that the detection is real with at least 99% confidence. We only detect O IV lambda788, O IV lambda554, O III lambda833, and He I lambda584 in absorbers with Lyalpha equivalent widths greater than or similar to0.6 Angstrom, which are likely associated with traditional metal line systems. We find no evidence in any subsamples for absorption from N IV lambda765, Ne V lambda568, Ne VI lambda559, Ne VIII lambdalambda770, 780, or Mg x lambdalambda610, 625. The measured equivalent widths of O V suggest values of in the range -1.7 to -0.6 for 10(13.2) cm(-2) < N(H I)less than or similar to 10(15) 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]approximate to -2.2 to 1.3. Comparing to studies of C IV we estimate [O/C] approximate to 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.