THE END OF HELIUM REIONIZATION AT z ≃ 2.7 INFERRED FROM COSMIC VARIANCE IN HST /COS He II Lyα ABSORPTION SPECTRA

We report on the detection of strongly varying intergalactic He ii absorption in HST /COS spectra of two z em ≃ 3 quasars. From our homogeneous analysis of the He ii absorption in these and three archival sightlines, we find a marked increase in the mean He ii effective optical depth from at z ≃ 2.3 to at z ≃ 3.2, but with a large scatter of at 2.7 < z < 3 on scales of ∼10 proper Mpc. This scatter is primarily due to fluctuations in the He ii fraction and the He ii -ionizing background, rather than density variations that are probed by the coeval H i forest. Semianalytic models of He ii absorption require a strong decrease in the He ii -ionizing background to explain the strong increase of the absorption at z ≳ 2.7, probably indicating He ii reionization was incomplete at z reion ≳ 2.7. Likewise, recent three-dimensional numerical simulations of He ii reionization qualitatively agree with the observed trend only if He ii reionization completes at z reion ≃ 2.7 or even below, as suggested by a large in two of our five sightlines at z < 2.8. By doubling the sample size at 2.7 ≲ z ≲ 3, our newly discovered He ii sightlines for the first time probe the diversity of the second epoch of reionization when helium became fully ionized.