AAS 199th meeting, Washington, DC, January 2002
Session 18. High Velocity Clouds and The Intergalactic Medium
Display, Monday, January 7, 2002, 9:20am-6:30pm, Exhibit Hall

## [18.05] He II and H I Absorption studies with FUSE and VLT

J.M. Shull, , J. Tumlinson (CU-CASA), M. Giroux (ETSU), D. Reimers (Hamburg), J. Kriss (STScI)

We provide an in-depth analysis of the He~II and H~I absorption in the intergalactic medium at z = 2.3-2.9 toward HE~2347-4342, using FUSE, Keck, and VLT spectra. Following up on the study of Kriss et al. (2001, Science, 293, 1112), our analysis focuses on the small-scale variability in the He~II/H~I ratios with redshift, which probably reflects the nature of the ionizing sources. We use photoionization models to understand the ionizing background and its possible changes at z < 2.7 arising from the overlap of helium ionization fronts from early sources. The He II Ly\alpha absorption is visible as small-scale fluctuations in absorption between 2.3 \leq z \leq 2.7. We compare the Keck and VLT data, including a Lyman Limit system at za = 2.75 (3420 Å). Many of the He~II features have H~I counterparts with log N(H~I) > 12.2, accounting for much of the observed He~II opacity. The ratio \eta = N(He~II)/N(H~I) ranges from 1 to >1000. Ratios of \eta < 100 are consistent with photoionization of the absorbing gas by a hard ionizing spectrum resulting from the quasar background, but ratios \eta > 100 in many locations indicate additional contributions from starburst galaxies or heavily filtered quasar radiation. The ionizing spectra may help to explain the difference between the reionization epochs of He~II (z \approx 2.8 ±0.2) and H~I (z \approx 6.2 ±0.2)