Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001aas...199.2902t&link_type=abstract
American Astronomical Society, 199th AAS Meeting, #29.02; Bulletin of the American Astronomical Society, Vol. 33, p.1353
Astronomy and Astrophysics
Astronomy
Scientific paper
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 α . 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 α EUV = -1.57 +/- 0.17 (fν ν α ), while for radio-loud QSOs the power-law index is α EUV = -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 α forest in the redshift range 1.7 < z < 2.9. Using Keck HIRES spectra to extract the position and strength of the Lyman α forest features, we generate composite absorption spectra from the FOS data to assess the strength of weak O V λ 630 and O IV λ 788 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 1013.6cm -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 1015cm -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.
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