Patchy intergalactic He ii absorption in HE 2347 − 4342 ⋆

We report on observations of redshifted He ii 303.8 ˚ A absorption in the high-redshift QSO HE 2347−4342 (z = 2.885, V = 16.1) with the Goddard High Resolution Spectrograph on board HST in its low resolution mode (△λ = 0.7 ˚ A). With f λ = 3.6 10 −15 erg cm −2 s −1Å −1 at the expected position of He ii 304Å absorption it is the most UV-bright high redshift QSO discovered so far. We show that the He ii opacity as a function of redshift is patchy showing spectral regions with low He ii opacity (" voids ") and regions with high He ii opacity (blacked-out " troughs ") and no detectable flux. Combination with high-resolution optical spectra of the Lyα forest using CASPEC at the 3.6 m telescope shows that the voids can be explained either exclusively by Lyα forest cloud absorption with a moderate N He ii /N H i ratio η ≤ 100 and turbulent line broadening or by a combination of Lyα forest with η = 45 and thermal broadening plus a diffuse medium with τ He ii GP ≈ 0.3. Since the latter is a minimum assumption for the Lyα forest, a strict upper limit to a diffuse medium is Ω diff < 0.02 h −1.5 50 at z = 2.8. In the troughs in addition to the Lyα forest opacity a continuous He ii 304Å opacity τ = 4.8 +∞ −2 is required. In case of photoionization, the troughs would require a diffuse component with a density close to Ω ≃ 0.077 (η/45) −0.5 h −1.5 50 , i.e. all baryons in the universe, which is inconsistent, however, with the observed absence of such a component in the voids. A tentative interpretation is that we observe the epoch of partial He ii reionization of the universe with patches not yet reionized. In that case a diffuse component −1 50 would be sufficient to explain the " trough " opacity. The size of the 1163–1172Å trough is ∼ 6 h −1 50 Mpc or ∼ 2300 km s −1 , respectively. We also discuss partially resolved He ii absorption of a high-ionization associated absorption system. Despite its high luminosity HE 2347−4342 does not show a He ii proximity effect. A possible reason is that the strong associated system shields the He ii ionizing continuum.