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 ( Kriss et al .
2001 , Science , 293 , 1112 ) , we focus here on two major topics : ( 1 ) small-scale variability ( \Delta z \approx 10 ^ { -3 } ) in the ratio \eta = N ( He II ) /N ( H I ) ; and ( 2 ) an observed correlation of high- \eta 
absorbers ( soft radiation fields ) with voids in the ( H I ) Ly \alpha 
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 < \tau _ { HeII } < 2.3 and 0.02 < \tau _ { HI } < 3.9 , and reliably determine values of \eta \approx 4 \tau _ { HeII } / \tau _ { HI } 
over the range 0.1 to 460 .
Values \eta = 20 - 200 are consistent with models of photoionization by quasars with observed spectral indices \alpha _ { s } = 0 - 3 .
Values \eta > 200 may require additional contributions from starburst galaxies , heavily filtered quasar radiation , or density variations .
Regions with \eta < 30 may indicate the presence of local hard sources .
We find that \eta is higher in “ void ” regions , where H I is weak or undetected and \sim 80 % of the path length has \eta > 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 } \sim 6.2 \pm 0.2 ) and He II ( z _ { HeII } \sim 2.8 \pm 0.2 ) .