Intergalactic HeII absorption towards QSO 1157+3143

We report the discovery of a further line of sight allowing detection of He II Ly alpha absorption by the intergalactic medium. A HST/STIS survey of 32 bright z similar to 3 quasars selected from the Hamburg Quasar Surveys yielded one detection toward QSO 1157+3143 ( z similar to 3, B similar or equal to 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 similar to 1700 angstrom to allow study of the He II absorption spectrum in the redshift range 2.75 <= z <= 2.97. The absorption is characterized by alternating voids and dense filament structures seen in both He II and H I. Attempts to model the He II opacity in terms of H I Ly alpha forest absorption are not successful in the voids, suggesting that He II reionization is not complete between z = 2.77 and 2.97 or that an optically thin Lyman limit system with z approximate to 0.3 is responsible for the additional opacity.