2004ApJ...600..570S

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 and the Ultraviolet-Visual Echelle Spectrograph on the Very Large Telescope. Following up on our earlier study, we focus here on two major topics: (1) small-scale variability (Δz~10^–3) in the ratio η=N(HeII)/N(HI) 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. Given the 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_/τHI_over the range 0.1-460. 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 active galactic nucleus 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).