Hubble Space TelescopeSpace Telescope Imaging System Observations of the HeiiGunn‐Peterson Effect toward HE 2347−4342

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Abstract

We present an HST Space Telescope Imaging System (STIS) spectrum of the He II Gunn-Peterson e ect 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 twodimensional detector also allows us to better characterize the sky and dark background.We conrm 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 in the high-opacity regions.We use the STIS spectrum qHe II together with a Keck-HIRES spectrum covering the corresponding H I Lya forest to calculate a onedimensional 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 nd that S is generally large but presents important variations, from D30 in the opacity gaps to a 1 p lower limit of 2300 at z ^2.86, in a region that shows an extremely low H I opacity over a 6.5 spectral range.We note that A 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.

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