The possible discovery of the epoch of He-reionization

We report on observations of redshifted He ii 303.8 Å absorption in the high-redshift QSO HE2347−4342 (z = 2.885, V = 16.1) with the Goddard High Resolution Spectrograph on board HST in its low resolution mode (4λ = 0.7 Å). With fλ = 3.6 10−15 erg cm−2 s−1 Å−1 at the expected position of He ii 304 Å 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 regionswith lowHe iiopacity (“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 3.6m telescope shows that the voids can be explained either exclusively by Lyα forest cloud absorption with a moderate NHe ii/NH i ratio η ≤ 100 and turbulent line broadening or by a combination of Lyα forestwith η =45 and thermal broadening plus a diffusemedium with τHe ii GP ≈ 0.3. Since the latter is a minimum assumption for the Lyα forest, a strict upper limit to a diffuse medium is Ωdiff < 0.02 h−1.5 50 at z = 2.8. In the troughs in addition to the Lyα forest opacity a continuous He ii 304 Å 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 withΩdiff ≥ 1.3 10−4 h−1 50 would be sufficient to explain the “trough” opacity. The size of the Send offprint requests to: D. Reimers, [email protected] ? Based on observations collected at the European Southern Observatory, La Silla, Chile (ESO No. 58.B–0116). Based on IUE observations collected at the ESA VILSPA ground station near Madrid, Spain. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by Aura, Inc., under NASA contract NAS 5–26 555. 1163–1172 Å trough is ∼ 6 h−1 50 Mpc or ∼ 2300 km s−1, respectively. We also discuss partially resolvedHe ii absorption of a highionization 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.