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 Ly \alpha forest absorbers in the redshift range 1.6 < z < 2.9 .
We successfully detect O v \lambda 630 in Ly \alpha absorbers throughout the 10 ^ { 13 } to 10 ^ { 16.2 } { cm ^ { -2 } } column density range .
For a sample of absorbers with 10 ^ { 13.2 } < 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 \lambda 630 equivalent width of 10.9 \pm 3.7 mÅ .
We estimate the detection is real with at least 99 % confidence .
We only detect O iv \lambda 788 , O iv \lambda 554 , O iii \lambda 833 , and He i \lambda 584 in absorbers with Ly \alpha equivalent widths \gtrsim 0.6 Å , which are likely associated with traditional metal-line systems .
We find no evidence in any subsamples for absorption from N iv \lambda 765 , Ne v \lambda 568 , Ne vi \lambda 559 , Ne viii \lambda \lambda 770 , 780 , or Mg x \lambda \lambda 610 , 625 .
The measured equivalent widths of O v suggest values of \langle O v/H i \rangle in the range -1.7 to -0.6 for 10 ^ { 13.2 } < N ( H i ) \lesssim 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 } ] \approx - 2.2 to -1.3 .
Comparing to studies of C iv , we estimate [ { O } / { C } ] \approx 0.3 to 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 .