Extreme-Ultraviolet Absorption Lines in Lyα Forest Absorbers and the Oxygen Abundance in the Intergalactic Medium

Abstract

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)<10^14.2 cm^-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 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.