2010ApJ...722.1312S

We report ultraviolet spectra of the high-redshift (z_em~ 2.9) quasar, HE 2347-4342, taken by the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope. 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 Lyα 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 signal-to-noise ratio (S/N), and better determined backgrounds than previous studies, with sensitivity to abundance fractions x_ He II_~ 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 and range from τ_ He II_≤ 0.02 to τ_He II_≥ 5, with a slow recovery in mean optical depth to He ≤ 2 at z < 2.7. The He II/H I optical-depth ratio varies (η~ 10-100 for 2.4 < z < 2.73 and η = 5-500 for 2.75 < z < 2.89) on scales Δz ≲ 0.01 (10.8 Mpc in comoving radial distance at z = 2.8), with numerous flux-transmission windows between 1135 and 1186 Å. The He II absorption extends to 1186.26 Å (z = 2.905), including associated absorbers with z_abs~ z_QSO and minimal "proximity effect" of flux transmission at the He II edge. We propose a QSO systemic redshift z_QSO= 2.904±0.002, some Δz = 0.019 higher than that derived from O I λ1302 emission. Three long troughs (4-10 Å or 25-60 Mpc comoving distance) of strong He II absorption between z = 2.75and2.90 are uncharacteristic of the intergalactic medium if He II reionized at z_r~ 3. Contrary to recent indirect estimates (z_r= 3.2±0.2) from H I optical depths, the epoch of He II reionization may extend to z ≲ 2.7.