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Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992hst..prop.3801t&link_type=abstract
HST Proposal ID #3801
Other
Hst Proposal Id #3801 Quasars &Amp, Agn
Scientific paper
Ultraviolet images will be obtained in snapshot mode of the 500 known high-redshift (z > 2.8) QSOs in order to identify the few (about 20) targets which have sufficient ultraviolet flux for subsequent FOC/FOS or GHRS observations of He II 304. The detection of absorption by the Helium II Lyman-alpha line at 304 A, one of the most exciting prospects of the HST, will provide the first direct detection of the diffuse intergalactic medium (IGM). The absence of Gunn-Peterson H I 1215 absorption shows that the IGM is hot and/or of very low density, thus He I 584 is not expected to be observable. He II 304--the most promising line--should be observable from three sources: the diffuse IGM, the discrete Ly-alpha clouds, and the much rarer metal line absorption systems. The Gunn-Peterson continuum optical depth is not well constrained by models (range 0.3-3000). The mere detection of only one QSO below 304 A would rule out many models, limiting the IGM density, temperature, and ionization mechanisms. Similarly the total absence of flux from several targets would rule out other models. Of the 500 targets, 40 should be bright enough for subsequent FOC imaging in bands above and below 304A, 10 for FOC prism spectra, and 3 for FOS spectra, which will allow measurement of the He II/H I ratio in Ly-alpha clouds and the spectral slope of the background ionizing radiation.
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