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Issue A&A
Volume 442, Number 1, October IV 2005
Page(s) 63 - 68
Section Cosmology (including clusters of galaxies)
DOI http://dx.doi.org/10.1051/0004-6361:20053365



A&A 442, 63-68 (2005)
DOI: 10.1051/0004-6361:20053365

Intergalactic HeII absorption towards QSO 1157+3143

D. Reimers1, C. Fechner1, H.-J. Hagen1, P. Jakobsen2, D. Tytler3 and D. Kirkman3

1  Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
    e-mail: cfechner@hs.uni-hamburg.de
2  Astrophysics Missions Division, RSSD, European Space Agency, ESTEC, 2200 AG Noordwijk, The Netherlands
3  Center for Astrophysics and Space Sciences, University of California, San Diego, MS 0424, La Jolla, CA 92093-0424, USA

(Received 4 May 2005/ Accepted 30 June 2005 )

Abstract
We report the discovery of a further line of sight allowing detection of $\ion{He}{ii}$ Ly$\alpha$ absorption by the intergalactic medium. A HST/STIS survey of 32 bright $z \sim 3$ quasars selected from the Hamburg Quasar Surveys yielded one detection toward QSO 1157+3143 ( $z \sim 3, B \simeq 17$). A 10 orbit follow-up spectrum reveals a UV spectrum significantly suppressed by two intervening Lyman limit systems at z=2.77 and 2.94, but with the continuum flux recovering sufficiently shortward of ~ $1700\,{\rm\AA}$ to allow study of the $\ion{He}{ii}$ absorption spectrum in the redshift range $2.75 \leq z \leq 2.97$. The absorption is characterized by alternating voids and dense filament structures seen in both $\ion{He}{ii}$ and $\ion{H}{i}$. Attempts to model the $\ion{He}{ii}$ opacity in terms of $\ion{H}{i}$ Ly$\alpha$ forest absorption are not successful in the voids, suggesting that $\ion{He}{ii}$ reionization is not complete between z=2.77 and 2.97 or that an optically thin Lyman limit system with $z \approx 0.3$ is responsible for the additional opacity.


Key words: galaxies: quasars: individual: QSO 1157+3143 -- galaxies: quasars: absorption lines -- cosmology: observations

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