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

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Patchy intergalactic He II absorption in HE 2347-4342 *

The possible discovery of the epoch of He-reionization

D. Reimers 1, S. Köhler 1, L. Wisotzki 1, D. Groote 1, P. Rodriguez-Pascual 2 and W. Wamsteker 2

1 Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany
2 ESA-Vilspa, PO Box 50727, E-28080 Madrid, Spain

Received 26 May 1997 / Accepted 16 July 1997


We report on observations of redshifted He II 303.8 Å absorption in the high-redshift QSO HE 2347-4342 ([FORMULA], [FORMULA]) with the Goddard High Resolution Spectrograph on board HST in its low resolution mode ([FORMULA] Å). With [FORMULA] 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 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 Ly [FORMULA] forest using CASPEC at the 3.6 m telescope shows that the voids can be explained either exclusively by Ly [FORMULA] forest cloud absorption with a moderate [FORMULA] ratio [FORMULA] and turbulent line broadening or by a combination of Ly [FORMULA] forest with [FORMULA] = 45 and thermal broadening plus a diffuse medium with [FORMULA] 0.3. Since the latter is a minimum assumption for the Ly [FORMULA] forest, a strict upper limit to a diffuse medium is [FORMULA] h [FORMULA] at [FORMULA].

In the troughs in addition to the Ly [FORMULA] forest opacity a continuous He II 304 Å opacity [FORMULA] is required. In case of photoionization, the troughs would require a diffuse component with a density close to [FORMULA] h [FORMULA], 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 with [FORMULA] h [FORMULA] would be sufficient to explain the "trough" opacity. The size of the 1163-1172 Å trough is [FORMULA] h [FORMULA] Mpc or [FORMULA] 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.

Key words: quasars: individual: HE 2347-4342 – quasars: absorption lines – cosmology: observations

* 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.

Send offprint requests to: D. Reimers,

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998