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Astron. Astrophys. 327, 890-900 (1997)

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1. Introduction

A longstanding goal of observational cosmology has been the detection of a diffuse intergalactic medium (IGM), suspected to contain a major fraction of the baryons of the universe produced in the big-bang, by means of the Gunn-Peterson (1965) effect. The test using H I Ly [FORMULA] has been negative so far (e.g. Steidel & Sargent 1987; Giallongo et al. 1992, 1994). The conclusion has been that the IGM is highly ionized and/or contains a much lower fraction of the baryons of the universe than originally expected. Recent hydrodynamical models of structure formation indeed predict that the fragmentation of baryons is nearly complete and that a diffuse IGM is suppressed by two orders of magnitude (Miralda-Escudé et al. 1996; Meiksin 1997).

With the launch of the Hubble Space Telescope it was hoped to detect a highly ionized diffuse IGM via the He II Ly [FORMULA] (303.8 Å) line, which was predicted to be much more sensitive than H I Ly [FORMULA].

The observation of absorption on the blue side of the He II 304 Å line in high-redshift QSOs has been pioneered by Jakobsen et al. (1994) who observed a completely absorbed spectrum ([FORMULA]) on the blue side of the redshifted He II line in Q 0302-003 ([FORMULA]). He II absorption in a second QSO, PKS 1935-692 ([FORMULA]), was discovered with HST by Tytler (1995), with a similar result (cf. Jakobsen 1996), while Davidsen et al. (1996) observed a lower He II opacity ([FORMULA] at [FORMULA]) toward HS 1700+6416 ([FORMULA])
with HUT/ASTRO-2.

Due to their low spectral resolution, all these observations did not allow to distinguish between a He II 304 Å forest and absorption by a diffuse medium. By means of model calculations using high resolution optical H I Ly [FORMULA] forest spectra, both Songaila et al. (1995) for Q 0302-003 and Davidsen et al. (1996) for HS 1700+6416 demonstrated that the He II opacity could be explained by the Ly [FORMULA] forest alone with [FORMULA] 80, a value roughly consistent with photoionization calculations using predicted metagalactic radiation fields due to QSOs with absorption by intergalactic matter taken into account (Meiksin & Madau 1993; Giroux et al. 1995; Haardt & Madau 1996). On the other hand, recent reobservations of Q 0302-003 with the GHRS on board of HST by Hogan et al. (1997) seem to indicate that a diffuse component with [FORMULA] 0.01 (h/0.7)-1.5 is required in order to explain the He II opacity in the well-known "void" (cf. Dobrzycki & Bechthold 1991) in the Ly [FORMULA] forest of Q 0302-003. All the HST observations of the intergalactic He II opacity obtained so far suffered from the faintness ([FORMULA]) of the QSOs. The difficulty in finding more suitable targets is best demonstrated by the fact that the FOC surveys for unabsorbed [FORMULA] QSOs by Jakobsen et al. and Tytler et al. detected only 2 moderate opacity lines of sight in more than 110 observed QSOs (cf. Jakobsen 1996). In this paper we report on the discovery of the extremely bright ([FORMULA]), [FORMULA] QSO HE 2347-4342 within the Hamburg/ESO survey for bright QSOs and on successful observations of He II 304 Å absorption with the GHRS onboard the Hubble Space Telescope in its low resolution mode. The combination of partially resolved He II absorption with high resolution optical spectra taken with CASPEC at the ESO 3.6 m telescope allows to constrain the [FORMULA] ratio in Ly [FORMULA] forest clouds and to quantify the contribution of a diffuse component to the He II absorption.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998