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Title:
Hubble Space Telescope Space Telescope Imaging System Observations of the He II Gunn-Peterson Effect toward HE 2347-4342
Authors:
Smette, Alain; Heap, Sara R.; Williger, Gerard M.; Tripp, Todd M.; Jenkins, Edward B.; Songaila, Antoinette
Affiliation:
AA(Laboratory for Astronomy and Solar Physics, NASA Goddard Space Flight Center, Code 681, Greenbelt, MD 2077; NOAO, P.O. Box 26732, Tucson, AZ 85716-6732; Chercheur qualifié, FNRS, Belgiu; Institut d'Astrophysique et de Géophysique, Université de Liège, B-4000 Liège, Belgium. ), AB(Laboratory for Astronomy and Solar Physics, NASA Goddard Space Flight Center, Code 681, Greenbelt, MD 20771 ), AC(Laboratory for Astronomy and Solar Physics, NASA Goddard Space Flight Center, Code 681, Greenbelt, MD 20771; NOAO, P.O. Box 26732, Tucson, AZ 85716-6732. ), AD(Princeton University Observatory, Princeton, NJ 08544 ), AE(Princeton University Observatory, Princeton, NJ 08544 ), AF(Institute for Astronomy, 2680 Woodlawn Drive, University of Hawaii, Honolulu, HI 96822 )
Publication:
The Astrophysical Journal, Volume 564, Issue 2, pp. 542-558. (ApJ Homepage)
Publication Date:
01/2002
Origin:
UCP
Astronomy Keywords:
Cosmology: Observations, Galaxies: Intergalactic Medium, Galaxies: Quasars: Absorption Lines, Galaxies: Quasars: Individual: Alphanumeric: HE 2347-4342
DOI:
10.1086/324397
Bibliographic Code:
2002ApJ...564..542S

Abstract

We present an HST Space Telescope Imaging System (STIS) spectrum of the He II Gunn-Peterson effect toward HE 2347-4342. Compared to the previous HST Goddard High Resolution Spectrograph data obtained by Reimers et al. the STIS spectrum has a much improved resolution. The two-dimensional detector also allows us to better characterize the sky and dark background. We confirm the presence of two spectral ranges of much reduced opacity, the opacity gaps, and provide improved lower limits on the He II Gunn-Peterson opacity τHeII in the high-opacity regions. We use the STIS spectrum together with a Keck-HIRES spectrum covering the corresponding H I Lyα forest to calculate a one-dimensional map of the softness S of the ionization radiation along the line of sight toward HE 2347-4342, where S is the ratio of the H I to He II photoionization rates. We find that S is generally large but presents important variations, from ~30 in the opacity gaps to a 1 σ lower limit of 2300 at z~=2.86, in a region that shows an extremely low H I opacity over a 6.5 Å spectral range. We note that a large softness parameter naturally accounts for most of the large Si IV/C IV ratios seen in other quasar absorption line spectra. We present a simple model that reproduces the shape of the opacity gaps in absence of large individual absorption lines. We extend the model described in Heap et al. to account for the presence of sources close to the line of sight of the background quasar. As an alternative to the delayed reionization model suggested by Reimers et al., we propose that the large softness observed at z~=2.86 is due to the presence of bright soft sources close to the line of sight, i.e., for which the ratio between the number of H I to He II ionizing photons reaching the intergalactic medium is large. We discuss these two models and suggest ways to discriminate between them. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.
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