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Astron. Astrophys. 327, 890-900

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Patchy intergalactic He II absorption in HE 2347-4342*

The possible discovery of the epoch of He-reionization

D. Reimers1 and S. Köhler1, and L. Wisotzki1, and D. Groote1, and P. Rodriguez-Pascual2, and W. Wamsteker2

1Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany
2ESA-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 (z = 2.885, V=16.1) with the Goddard High Resolution Spectrograph on board HST in its low resolution mode ($\bigtriangleup\lambda= 0.7$ Å). With $f_{\lambda}=3.6\ 10^{-15}$ 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 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$\alpha$ forest using CASPEC at the 3.6 m telescope shows that the voids can be explained either exclusively by Ly$\alpha$ forest cloud absorption with a moderate $N_{\mbox{\scriptsize He {\sc ii}}}/N_{\mbox{\scriptsize H {\sc i}}}$ ratio $\eta\leq 100$ and turbulent line broadening or by a combination of Ly$\alpha$ forest with $\eta$ = 45 and thermal broadening plus a diffuse medium with $\tau_{\mbox{\scriptsize GP}}^{\mbox{\scriptsize He {\sc ii}}} \approx$ 0.3. Since the latter is a minimum assumption for the Ly$\alpha$ forest, a strict upper limit to a diffuse medium is $\Omega_{\mbox{\scriptsize diff}}<0.02$ h50-1.5 at z = 2.8.

In the troughs in addition to the Ly$\alpha$ forest opacity a continuous He II 304 Å opacity $\tau = 4.8^{+\infty}_{-2}$ is required. In case of photoionization, the troughs would require a diffuse component with a density close to $\Omega
\simeq 0.077 (\eta/45)^{-0.5}$ h50-1.5, 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 $\Omega_{\mbox{\scriptsize diff}}\geq 1.3\ 10^{-4}$ h-150 would be sufficient to explain the ''trough'' opacity. The size of the 1163-1172 Å trough is $\sim 6$ h50-1 Mpc or $\sim 2300$ 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.

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, dreimers@hs.uni-hamburg.de

© European Southern Observatory (ESO) 1997

Online publication: October 20, 1997
Last change: April 6, 1998