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Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997hst..prop.7272h&link_type=abstract
HST Proposal ID #7272
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Hst Proposal Id #7272 Quasar Absorption Lines
Scientific paper
The first paragraph below is the OLD abstract. Please see the 2nd paragraph below for a revision in response to a NASA ruling. We propose to reobserve with STIS three quasars displaying He+ Lyman-Alpha absorption. For HE2347-4342, a long duration STIS G140M observation will substantially improve the spectral resolution over extant and planned GHRS/STIS low resolution observations, and provide a much more definitive measurement of the cloud contribution to the helium absorption. For Q0302-003, a short duration STIS G230L spectrum will permit an accurate measure of the near-UV continuum, study of the He I Lyman-alpha forest clouds {clouds also causing some of the He II absorption}, and permit a search for intervening Ly-limit HI systems in a previously unexplored wavelength {redshift} regime. For PKS 1935-692, a short duration long-slit G140L STIS spectrum will provide an independent and secure estimate of the flux zero-level to compare with long-exposure GHRS spectra taken by Tytler et al., and ultimately permit a detailed intercomparison amongst the various data sets on all three objects. Together with groundbased data on the quasars and their hydrogen Lyman Alpha absorbers, the proposed STIS observations will allow sufficiently detailed information on the protogalactic medium to confirm quantitatively the inferences already suggested from low resolution GHRS data, extend them to higher resolution in the HeII region, include He I cloud comparisions, allow detailed comparisons of inferences along different sightlines with high confidence. These inferences include estimates of the relative contributions of discrete clouds and diffuse gas to absorption near and far from the quasar, the intensity of the ionizing background at the He^+ edge, and the total density of diffuse protogalactic gas at z 3, at a level of sensitivity which significantly and uniquely constrains models of the cosmic ionization, structure formation, and baryon distribution. We will waive all proprietary rights to the data. After a NASA ruling that the above HE2347 observations are in conflict with a STIS/GTO program, we were requested to substitute another target instead. In this revised program, visits 1 thru 4 now constitute a long exposure with G140L on PKS1935, and will consume the bulk of our allotted orbits, if this conflict-ruling on HE2347 is upheld. These STIS PKS1935 will provide substantially improved S/N and substntially reduced geocoronal contamination compared with extant GHRS spectra, providing a much more secure measure of both discrete cloud and diffuse IGM contributions to the HeII opacity in this sightline.
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