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We report the discovery of a further line of sight allowing detection of HeIILy alpha absorption by the intergalactic medium. A HST/STIS survey of 32 brightz ~ 3 quasars selected from the Hamburg Quasar Surveys yielded one detectiontoward QSO 1157+3143 (z ~ 3, B ~ 17). A 10 orbit follow-up spectrum reveals aUV spectrum significantly supressed by two intervening Lyman limit systems atz=2.77 and 2.94, but with the continuum flux recovering sufficiently shortwardof ~ 1700 A to allow study of the HeII absorption spectrum in the redshiftrange 2.75 < z < 2.97. The absorption is characterized by alternating voids anddense filament structures seen in both HeII and HI. Attempts to model the HeIIopacity in terms of HI Ly alpha forest absorption are not successful in thevoids, suggesting that HeII reionization is not complete between z=2.77 and2.97 or that an optically thin Lyman limit system with z ~ 0.3 is responsiblefor the additional opacity.Comment: 6 pages, 4 figures, accepted by A&

Intergalactic HeII absorption towards QSO 1157+3143