We report ultraviolet spectra of the high-redshift ( z _ { em } \approx 2.9 ) quasar , HE 2347 - 4342 , taken by the Cosmic Origins Spectrograph ( COS ) on the Hubble Space Telescope ( HST ) .
Spectra in the G130M ( medium-resolution , 1135–1440 Å ) and G140L ( low-resolution , 1030–2000 Å ) gratings exhibit patchy Gunn-Peterson absorption in the 303.78 Å Ly \alpha line of He II between z = 2.39–2.87 ( G140L ) and z = 2.74–2.90 ( G130M ) .
With COS , we obtain better spectral resolution , higher-S/N , and better determined backgrounds than previous studies , with sensitivity to abundance fractions x _ { HeII } \approx 0.01 in filaments of the cosmic web .
The He ii optical depths from COS are higher than those with the Far Ultraviolet Spectroscopic Explorer ( FUSE ) and range from \tau _ { HeII } \leq 0.02 to \tau _ { HeII } \geq 5 , with a slow recovery in mean optical depth to \langle \tau _ { HeII } \rangle \leq 2 at z < 2.7 .
The He ii/H i optical-depth ratio varies ( \eta \approx 10–100 for 2.4 < z < 2.73 , and \eta = 5–500 for 2.75 < z < 2.89 ) on scales \Delta z \lesssim 0.01 ( 10.8 Mpc in comoving radial distance at z = 2.8 ) , with numerous flux-transmission windows between 1135–1186 Å .
The He ii absorption extends to 1186.26 Å ( z = 2.905 ) , including associated absorbers with z _ { abs } \approx z _ { QSO } and minimal “ proximity effect ” of flux transmission at the He ii edge .
We propose a QSO systemic redshift z _ { QSO } = 2.904 \pm 0.002 , some \Delta z = 0.019 higher than that derived from O i \lambda 1302 emission .
Three long troughs ( 4–10 Å or 25–60 Mpc comoving distance ) of strong He ii absorption between z = 2.75 - 2.90 are uncharacteristic of the intergalactic medium if He ii reionized at z _ { r } \approx 3 .
Contrary to recent indirect estimates ( z _ { r } = 3.2 \pm 0.2 ) from H i optical depths , the epoch of He ii reionization may extend to z \lesssim 2.7 .