11286(  1) - 03/30/07 11:30  - [  1]

                  HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 11286

Version: 1    Check-in Time: 30-Mar-2007 15:30:25       STScI Edit Number: 0

Title
Lyman continuum Absorption and the IGM Opacity at low Redshifts
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Type             Cycle     Parallel Pointing Tolerance
AR               16        <none>
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Investigators
                                                                                     Contact?
    PI: Dr. David R. Tytler               University of California - San Diego
   CoI: Dr. David Kirkman                 University of California - San Diego          N

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Abstract

We will measure the amount of Lyman continuum absorption in the universe at 0.5 < z
< 2.5 three times more accurately than currently published results, which have
errors of 30%. We will work with FOS high dispersion spectra of 134 QSOs, FOS low
dispersion spectra of 42 QSOs, STIS low dispersion spectra of 79 QSOs and ACS
spectra of 36 QSOs. We expect to detect approximately 155 Lyman limit absorption
systems (LLS) towards these QSOs. Published work uses approximately 17 LLS at these
redshifts from HST spectra. The list of LLS have many uses, since these systems are
amongst the best places to measure ionization and abundances, they include likely
COS targets. We will derive the LLS density, per unit redshift, which is a key input
to calculations of the intensity of the cosmic UV background. We use the optical
depths in the Lyman continuum, and fits to Lyman series in the best spectra, to
obtain the distribution of the H I column densities of the LLS, and especially the
partial LLS with optical depth < 1. The UV background is required to understand the
ionization of the intergalactic medium and the gas in the outer regions of galaxies
that causes QSO absorption lines. It is also needed to run realistic simulations
which are the key to understanding the IGM and QSO absorption systems.
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