STIS observations of He II Gunn-Peterson absorption toward Q0302-003

The ultraviolet spectrum (1145-1720 Å) of the distant quasar Q0302-003 (z = 3.286) was observed at 1.8 Å 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 Lyα 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 Lyα forest (provided by A. Songaila and by M. Rauch and W. Sargent). We find that near the quasar, He II Lyα 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 Lyα opacity at z ≈ 3.15 is τ ≥ 4.8. In the Dobrzycki-Bechtold void in the H I Lyα forest near z = 3.18, the average He II opacity τ = 4.47^+0.48-_0.33. 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 = Γ^J_{H I}/Γ^J_{He II} ≃ 800, indicating a significant stellar contribution. At z = 3.05, there is a distinct region of high He II Lyα 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 Lyα opacity detected by STIS, τ = 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.