The End of Helium Reionization at z~2.7 Inferred from Cosmic Variance in HST/COS HeII Lyman Alpha Absorption Spectra

Understand

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.