PATCHY INTERGALACTIC HE-II ABSORPTION IN HE-2347-4342 - THE POSSIBLE DISCOVERY OF THE EPOCH OF HE-REIONIZATION

We report on observations of redshifted He II 303.8 Angstrom absorptio n 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 resolutio n mode (Delta lambda = 0.7 Angstrom). With f(lambda) = 3.6 10(-15) erg cm(-2) s(-2) Angstrom(-1) at the expected position of He II 304 Angst rom absorption it is the most UV-bright high redshift QSO discovered s o far. We show that the He II opacity as a function of redshift is pat chy showing spectral regions with low He II opacity (''voids'') and re gions with high He II opacity (blacked-out ''troughs'') and no detecta ble flux. Combination with high-resolution optical spectra of the Ly a lpha forest using CASPEC at the 3.6 m telescope shows that the voids c an be explained either exclusively by Ly alpha forest cloud absorption with a moderate N-He ii/N-H i ratio eta less than or equal to 100 and turbulent line broadening or by a combination of Ly alpha forest with eta = 45 and thermal broadening plus a diffuse medium with tau(GP)(He ii) approximate to 0.3. Since the latter is a minimum assumption for the Ly alpha forest, a strict upper limit to a diffuse medium is Omega (diff) < 0.02 h(50)(-1.5) at z = 2.8. In the troughs in addition to th e Ly alpha forest opacity a continuous He II 304 Angstrom opacity tau = 4.8(-2)(+infinity) is required. In case of photoionization, the trou ghs would require a diffuse component with a density close to Omega si milar or equal to 0.077 (eta/45)(-0.5) h(50)(-1.5), i.e. all baryons i n the universe, which is inconsistent, however, with the observed abse nce of such a component in the voids. A tentative interpretation is th at we observe the epoch of partial He rr reionization of the universe with parches not yet reionized. In that case a diffuse component with Omega(diff) greater than or equal to 1.3 10(-4) h(50)(-1) would be suf ficient to explain the ''trough'' opacity. The size of the 1163-1172 A ngstrom trough is similar to 6 h(50)(-1) Mpc or similar to 2300 km s(- 1), respectively. We also discuss partially resolved He II absorption of a high-ionization associated absorption system. Despite its high lu minosity HE 2347-4342 does not show a He II proximity effect. A possib le reason is that the strong associated system shields the He II ioniz ing continuum.