paper

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

arXiv:astro-ph/0207301 · doi:10.1086/342836

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 LyA forest absorbers in the redshift range 1.6 < z < 2.9. We successfully detect O V 630 in LyA 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 630 equivalent width of 10.9 +/- 3.7 mA. We estimate the detection is real with at least 99% confidence. We only detect O IV 788, O IV 554, O III 833, and HeI 584 in absorbers with LyA equivalent widths > 0.6 A, which are likely associated with traditional metal-line systems. We find no evidence in any subsamples for absorption from N IV 765, NeV 568, NeVI 559, NeVIII 770, 780, or MgX 610, 625. The measured equivalent widths of O V suggest values of <O V / H I> in the range -1.7 to -0.6 for 10^13.2 < N(H I) < 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] around -2.2 to -1.3. Comparing to studies of C IV, we estimate [O / C] around 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.

Accepted for publication in Nov 10, 2002 ApJ

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