Other
Scientific paper
Sep 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002phdt.........7t&link_type=abstract
Thesis (PhD). THE JOHNS HOPKINS UNIVERSITY, Source DAI-B 63/03, p. 1396, Sep 2002, 123 pages.
Other
Scientific paper
We use a sample of 332 Hubble Space Telescope spectra of 184 quasi-stellar objects (QSOs) with redshifts of z > 0.33 to study their typical extreme ultraviolet (EUV) spectral properties. We find that the continuum emission of QSOs between 500 and 1200 Å can be described by a power law with a typical index of αEUV ≈ -1.57 (Fν ∝ να ) for radio-quiet QSOs and αEUV ≈ -1.96 for radio-loud QSOs. Using the spectra from four of our sample QSOs, we create stacked composite absorption spectra to search for weak EUV absorption lines associated with intervening absorbers in the intergalactic medium (IGM) in the redshift range 1.6 < z < 2.9. We successfully detect O V λ630 in subsamples of Lyα absorbers throughout the 1013 to 1016.2 cm-2 range of H I column densities 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. Using photoionization models, we conclude that the lack of detectable O IV absorption except in the strongest absorption systems suggests a hard ultraviolet background, in agreement with measurements of the ratio of He II to H I in the IGM. The required background source spectrum is consistent with the EUV continuum shape of QSOs that we have derived, suggesting that photoionization by QSOs is the dominant ionization mechanism for the IGM at z ˜ 2 to 3. We estimate that the oxygen abundance relative to hydrogen in the IGM is around 10-1.5 to 10-1.9 times the standard solar abundance ratio. Comparing to studies of carbon, this implies that oxygen is a factor of 4 to 10 times more abundant than carbon with respect to the solar abundances. This relative overabundance of oxygen is consistent with other estimates of chemical abundances in the early universe and implies enrichment of the IGM by the products of high-mass stars.
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