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
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 (, ) with the Goddard High Resolution Spectrograph on board HST in its low resolution mode ( Å). With 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 forest using CASPEC at the 3.6 m telescope shows that the voids can be explained either exclusively by Ly forest cloud absorption with a moderate ratio and turbulent line broadening or by a combination of Ly forest with = 45 and thermal broadening plus a diffuse medium with 0.3. Since the latter is a minimum assumption for the Ly forest, a strict upper limit to a diffuse medium is h at .
In the troughs in addition to the Ly forest opacity a continuous He II 304 Å opacity is required. In case of photoionization, the troughs would require a diffuse component with a density close to h , 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 h would be sufficient to explain the "trough" opacity. The size of the 1163-1172 Å trough is h Mpc or 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.
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© European Southern Observatory (ESO) 1997
Online publication: April 6, 1998