STIS observations of HeII Gunn-Peterson absorption toward Q0302-003

The ultraviolet spectrum (1145-1720 Angstrom) of the distant quasar Q0302-003 (z = 3.286) was observed at 1.8 Angstrom resolution with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. A total integration time of 23,280 s was obtained. The spectrum clearly delineates the Gunn-Peterson He II absorption trough, produced by He II Ly alpha along the line of sight over the redshift range z = 2.78-3.28. Its interpretation was facilitated by modeling based on Reck HIRES spectra of the H I Ly alpha forest (provided by A. Songaila and by M. Rauch and W. Sargent). We find that near the quasar, He II Ly alpha absorption is produced by discrete clouds, with no significant diffuse gas; this is attributed to a He II "proximity effect" in which the quasar fully ionizes He in the diffuse intergalactic medium, but not the He in denser clouds. By two different methods we calculate that the average He II Ly alpha opacity at z approximate to 3.15 is tau greater than or equal to 4.8. In the Dobrzycki-Bechtold void in the H I Ly alpha forest near z = 3.18, the average He II opacity z = 4.47(-0.33)(+0.48). Such large opacities require the presence of a diffuse gas component as well as a soft UV background spectrum, whose softness parameter, defined as the ratio of the photoionization rate in H I over the one in He II S = Gamma(H I)(J)/Gamma(He II)(J) similar or equal to 800, indicating a significant stellar contribution. At z = 3.05, there is a distinct region of high He II Ly alpha transmission that most likely arises in a region where helium is doubly ionized by a discrete local source, quite possibly an AGN. At redshifts z < 2.87, the He II Ly alpha opacity detected by STIS, tau = 1.88, is significantly lower than at z > 3. Such a reduction in opacity is consistent with Songaila's report that the hardness of the UV background spectrum increases rapidly from z = 3 to z = 2.9.