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HST/STIS Observations of the He II Gunn-Peterson Effect

We present an HST STIS spectrum of the He II Gunn-Peterson effect towards HE 2347-4342. Compared to the previous HST GHRS data obtained by Reimers et al. (1997), the STIS spectrum has a much improved resolution. The 2-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 GunnPeterson opacity τHeII in the high opacity regions. We use the STIS spectrum together with a Keck–HIRES spectrum covering the corresponding H I Ly-α forest to calculate a 1-dimensional map of the softness S of the ionization radiation along the line of sight towards 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 which 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 to 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. (2000) 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. (1997), 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. Subject headings: cosmology: observations – galaxies: quasars: absorption lines – galaxies: intergalactic medium – QSOs: HE 2347–4342