The Fluctuating Intergalactic Radiation Field at Redshifts z = 2.3-2.9 from He II and H I Absorption toward HE 2347–4342

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(He )/N(H ) 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 < 2.3 and 0.02 < τ < 3.9 and reliably determine values of η ≈ 4τ/τ 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 ~ 6.2 ± 0.2) and He II (z ~ 2.8 ± 0.2).