Absorption due to He II Lya has now been detected at low resolution in the spectra of four quasars between redshifts z \ 2.74 and z \ 3.29. We assess these observations, giving particular attention to the radiative transfer of the ionizing background radiation, cloud di †use emission and ionization physics, and statistical Ñuctuations. We use high-resolution observations of H I absorption toward quasars to derive an improved model for the opacity of the intergalactic medium (IGM) from the distribution of absorbing clouds in column density and redshift. We use these models to calculate the H I and He II photoionization history, the ratio g \ He II/H I in both optically thin and self-shielded clouds, and the average line-blanketing contribution of the clouds to He II absorption. The derived ionization rate, s~1 (z \ 2È4), is consistent with the ionizing background intensity inferred from the ! H I \ (1È3) ] 10~12 "" proximity e †ect,ÏÏ but it remains larger than that inferred by N-body hydrodynamic simulations of the Lya absorber distribution. The He II observations are consistent with line blanketing from clouds having cm~2, although a contribution from a more di †use IGM would help to explain the observed N H I º 1012 opacity. We compute the expected He II optical depth, and examine the implications of the q He II (z), sizable Ñuctuations that arise from variations in the cloud numbers and ionizing radiation Ðeld. We assess how He II absorption constrains the intensity and spectrum of the ionizing radiation and the fractional contributions of the dominant sources (quasars and starburst galaxies). Finally, we demonstrate how high-resolution ultraviolet observations can distinguish between absorption from the di †use IGM and the Lya forest clouds and determine the source of the ionizing background.

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We provide an in-depth analysis of the He II and H I absorption in the intergalactic medium (IGM) at redshifts z = 2.3-2.9 toward HE 2347-4342, using spectra from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Ultraviolet-Visual Echelle Spectrograph (UVES) on the VLT telescope. Following up on our earlier study , Science, 293, 1112, we focus here on two major topics: (1) small-scale variability (∆z ≈ 10 −3 ) in the ratio η = N(He II)/N(H I); and (2) an observed correlation of high-η absorbers (soft radiation fields) with voids in the (H I) Lyα distribution. These effects may reflect fluctuations in the ionizing sources on scales of 1 Mpc, together with radiative transfer through a filamentary IGM whose opacity variations control the penetration of 1-5 ryd radiation over 30-40 Mpc distances. Owing to photon statistics and backgrounds, we can measure optical depths over the ranges 0.1 < τ HeII < 2.3 and 0.02 < τ HI < 3.9, and reliably determine values of η ≈ 4τ HeII /τ HI over the range 0.1 to 460. Values η = 20 − 200 are consistent with models of photoionization by quasars with observed spectral indices α s = 0 − 3. Values η > 200 may require additional contributions from starburst galaxies, heavily filtered quasar radiation, or density variations. Regions with η < 30 may indicate the presence of local hard sources. We find that η is higher in "void" regions, where H I is weak or undetected and ∼80% of the path length has η > 100. These voids may be ionized by local soft sources (dwarf starbursts) or by QSO radiation softened by escape from the AGN cores or transfer through the "cosmic web". The apparent differences in ionizing spectra may help to explain the 1.45 Gyr lag between the reionization epochs of H I (z HI ∼ 6.2 ± 0.2) and He II (z HeII ∼ 2.8 ± 0.2).

We obtained high-resolution FUSE (R ∼ 20,000) and VLT (R ∼ 45,000) spectra of the quasar HE2347-4342 to study the properties of the intergalactic medium between redshifts z = 2.0 − 2.9. The high-quality optical spectrum allows us to identify approximately 850 H I absorption lines with column densities between N ∼ 5 × 10 11 and 10 18 cm −2 . The reprocessed FUSE spectrum extends the wavelength coverage of the He II absorption down to an observed wavelength -2of 920Å. Source flux is detected to restframe wavelengths as short as ∼ 237Å. Approximately 1400 He II absorption lines are identified, including 917 He II Lyα systems and some of their He II Lyβ, Lyγ, and Lyδ counterparts. The ionization structure of He II is complex, with approximately 90 absorption lines that are not detected in the hydrogen spectrum. These features may represent the effect of soft ionizing sources. The ratio η=N(He II)/N(H I) varies approximately from unity to more than a thousand, with a median value of 62 and a distribution consistent with the intrinsic spectral indices of quasars. This provides evidence that the dominant ionizing field is from the accumulated quasar radiation, with contributions from other soft sources such as star-forming regions and obscured AGN, which do not ionize helium. We find an evolution in η toward smaller values at lower redshift, with the gradual disappearance of soft components. At redshifts z > 2.7, the large but finite increase in the He II opacity, τ = 5 ± 1, suggests that we are viewing the end stages of a reionization process that began at an earlier epoch. Fits of the absorption profiles of unblended lines indicate comparable velocities between hydrogen and He + ions. For line widths b He + = ξb H , we find ξ = 0.95 ± 0.12, indicating a velocity field in the intergalactic medium dominated by turbulence. At hydrogen column densities N < 3 × 10 12 cm −2 the number of forest lines shows a significant deficit relative to a power law, and becomes negligible below N = 10 11 cm −2 .