Intergalactic HeII absorption towards QSO 1157+3143

We report the discovery of a further line of sight allowing detection of $\ion{He}{ii}$ Lyα 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 $\ion{H}{i}$ Lyα 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.