He II REIONIZATION AND SOURCES OF METAGALACTIC IONIZATION

Intergalactic Lyα 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 Lyα (“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 (Lyα) lines, allow us to fix the epoch of helium reionization at zHeII ≈ 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 (η) exhibits variations over 1 Mpc distance scales (∆z ≈ 10). Intriguingly, this η-ratio is also correlated with Lyα 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 epochs. 1. Overview: Reionization of the Intergalactic Medium Before beginning a discussion of the FUSE contributions to the study of He II reionization, I provide an overview of the cosmological significance of “reionization”. This term refers to the sudden increase in ionization fraction of hydrogen gas in the intergalactic medium (IGM), initiated at redshifts z ≥ 6 by the first sources of ionizing radiation. After the epoch of recombination at zrec ≈ 1300, the IGM entered a long period of “dark ages” when the IGM was mostly neutral. This period ended sometime before z ≈ 6, perhaps as early as z = 10− 20. The ionization state of the IGM affects the transmission of radiation through the Lyα absorbers (H I and He II), as well as the electron-scattering optical depth to the cosmic microwave background (CMB) radiation. As individual ionizing sources (stars, galaxies, quasars) began emitting ionizing radiation, regions of ionized gas were produced, preceded by ionization fronts (I-fronts) in the IGM. These isolated ionized zones (H II regions, He III regions) eventually overlapped (Gnedin 2000), after which the IGM became transparent to Lyα radiation. Residual absorption continued as a “picket fence” of discrete, redshifted Lyα absorbers, often termed the “Lyα forest” – see spectra in Songaila (1998).