The End of Helium Reionization at z~2.7 Inferred from Cosmic Variance in   HST/COS HeII Lyman Alpha Absorption Spectra
  We report on the detection of strongly varying intergalactic HeII absorption in HST/COS spectra of two z~3 quasars. From our homogeneous analysis of the HeII absorption in these and three archival sightlines, we find a marked increase in the mean HeII effective optical depth from tau~1 at z~2.3 to tau>5 at z~3.2, but with a large scatter of 2< tau <5 at 2.7< z <3 on scales of ~10 proper Mpc. This scatter is primarily due to fluctuations in the HeII fraction and the HeII-ionizing background, rather than density variations that are probed by the co-eval HI forest. Semianalytic models of HeII absorption require a strong decrease in the HeII-ionizing background to explain the strong increase of the absorption at z>2.7, probably indicating HeII reionization was incomplete at z>2.7. Likewise, recent three-dimensional numerical simulations of HeII reionization qualitatively agree with the observed trend only if HeII reionization completes at z=2.7 or even below, as suggested by a large tau>3 in two of our five sightlines at z<2.8. By doubling the sample size at 2.7< z <3, our newly discovered HeII sightlines for the first time probe the diversity of the second epoch of reionization when helium became fully ionized. 
G\'abor Worseck (1), J. Xavier Prochaska (1), Matthew McQuinn (2),   Aldo Dall'Aglio (3), Cora Fechner (4), Joseph F. Hennaw...
astro-ph.CO