Intergalactic HeII absorption towards QSO 1157+3143

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  Lyα absorption by the intergalactic medium. A HST/STIS survey of 32 bright quasars selected from the Hamburg Quasar Surveys yielded one detection toward QSO 1157+3143 (). A 10 orbit follow-up spectrum reveals a UV spectrum significantly suppressed by two intervening Lyman limit systems at and 2.94, but with the continuum flux recovering sufficiently shortward of ~ to allow study of the  absorption spectrum in the redshift range . The absorption is characterized by alternating voids and dense filament structures seen in both  and . Attempts to model the  opacity in terms of  Lyα forest absorption are not successful in the voids, suggesting that  reionization is not complete between and 2.97 or that an optically thin Lyman limit system with is responsible for the additional opacity.