He II Reionization and Sources of Metagalactic Ionization
  Intergalactic Ly-alpha opacity suggests that H I was reionized at z ~ 6, while He II reionization was delayed to z ~ 3. Both epochs are in conflict with inferences from CMB optical depth (WMAP) which suggest early reionization at z = 10-20. One of the major contributions of FUSE to cosmological studies has been the detection of He II Ly-alpha (Gunn-Peterson) absorption in the spectra of AGN at redshifts z > 2.03. Spectra of He II absorbers, taken in concert with corresponding H I (Ly-alpha) lines, allow us to fix the epoch of helium reionization at z(HeII) ~ 2.8 +/- 0.2. Here, I review FUSE observations of He II absorption, together with their implications for the sources and transport of ionizing radiation over 30-50 Mpc distances through the IGM. FUSE observations of He II absorption toward HE 2347-4342, combined with Keck and VLT observations of H I, are consistent with photoionization by QSOs, with a wide range of intrinsic spectral indices, and modified by filtering and reprocessing in the IGM. The He II/H I ratio (eta) exhibits variations over 1 Mpc distance scales (Delta z ~ 0.001). Intriguingly, this eta-ratio is also correlated with Ly-alpha filaments and voids. The ionizing radiation field appears to be softer (higher He II/H I) in the voids. These void regions may be ionized by local soft sources (dwarf starburst galaxies), or the QSO radiation may softened by escape from AGN cores and transport through denser regions in the cosmic web. The differences in ionizing spectra may explain the 1.4 Gyr lag between H I and He II reionization epochs. 
J. Michael Shull (Univ. of Colorado)