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R. Telfer (Johns Hopkins Univ.)
We use a sample of 332 archival Hubble Spece Telescope spectra of 184 QSOs with z > 0.33, mostly from the Faint Object Spectrograph (FOS), to characterize the spectral properties of QSOs in the rest-frame extreme ultraviolet (EUV), concentrating in particular on the continuum shape at wavelengths shortward of Lyman \alpha. We find that the continuum of the mean composite of radio-quiet QSOs between 500 and 1200 Å\ can be well-described by a power law with index \alphaEUV = -1.57 ±0.17 (f\nu \propto \nu \alpha), while for radio-loud QSOs the power-law index is \alphaEUV = -1.96 ±0.12. The extrapolation of the EUV power-law for radio-quiet QSOs is consistent with soft X-ray data, suggesting a common EUV / X-ray continuum in these objects which is likely to be a large if not dominant ionization source for the intergalactic medium (IGM).
A few spectra in our sample provide data of sufficient quality and wavelength coverage to study EUV absorption lines associated with the Lyman \alpha forest in the redshift range 1.7 < z < 2.9. Using Keck HIRES spectra to extract the position and strength of the Lyman \alpha forest features, we generate composite absorption spectra from the FOS data to assess the strength of weak O~V \lambda630 and O~IV \lambda788 features as a function of H~I column density. We find that O~V is detected in groups of Lyman forest absorbers with median H~I column densities down to 1013.6{\rm cm -2}, corresponding to gas with a density only slightly above the universal mean. The weaker O~IV absorption is only evident in higher column density gas, down to around 1015{\rm cm -2}. We use these data along with models of the photoionization of the IGM by QSO radiation to place contraints on the oxygen abundance and ionization state of the IGM.