STIS Observations of He II Gunn-Peterson Absorption toward Q0302–003*

The ultraviolet spectrum (1145¨1720 of the distant quasar Q0302(003 (z \ 3.286) was observed at Ae ) 1.8 resolution with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope .A Ae total integration time of 23,280 s was obtained. The spectrum clearly delineates the Gunn-Peterson He II absorption trough, produced by He II Lya along the line of sight over the redshift range z \ 2.78¨3.28. Its interpretation was facilitated by modeling based on Keck HIRES spectra of the H I Lya forest (provided by A. Songaila and by M. Rauch and W. Sargent). We —nd that near the quasar, He II Lya absorption is produced by discrete clouds, with no signi—cant diUuse gas; this is attributed to a He II ii proximity eUect ˇˇ in which the quasar fully ionizes He in the diUuse intergalactic medium, but not the He in denser clouds. By two diUerent methods we calculate that the average He II Lya opacity at z B 3.15 is q " 4.8. In the Dobrzycki-Bechtold void in the H I Lya forest near z \ 3.18, the average He II opacity Such large opacities require the presence of a diUuse gas component as well as a q \ 4.47 ~0.330.48. soft UV background spectrum, whose softness parameter, de—ned as the ratio of the photoionization rate in H I over the one in He II