HST/COS OBSERVATIONS OF THE QUASAR HE 2347-4342: PROBING THE EPOCH OF He II PATCHY REIONIZATION AT REDSHIFTS z = 2.4-2.9

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 A) and G140L (low resolution, 1030-2000 A) gratings exhibit patchy Gunn-Peterson absorption in the 303.78 A Lya 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 tHe II [?] 0.02 to tHe II [?] 5, with a slow recovery in mean optical depth to [?]tHe II [?] [?] 2 at z < 2.7. The He II/H I optical-depth ratio varies (e[?] 10-100 for 2.4 < z < 2.73 and e = 5-500 for 2.75 < z < 2.89) on scales Dz [?] 0.01 (10.8 Mpc in comoving radial distance at z = 2.8), with numerous flux-transmission windows between 1135 and 1186 A. The He II absorption extends to 1186.26 A (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 Dz = 0.019 higher than that derived from O I l1302 emission. Three long troughs (4-10 A 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 zr [?] 3. Contrary to recent indirect estimates (zr = 3.2 +- 0.2) from H I optical depths, the epoch of He II reionization may extend to z [?] 2.7.