Early and Extended Helium Reionization Over More Than 600 Million Years of Cosmic Time

Understand

We measure the effective optical depth of HeII Ly\alpha\ absorption \tau$_\mathsf{eff,HeII}$ at 2.3<z<3.5 in 17 UV-transmitting quasars observed with UV spectrographs on the Hubble Space Telescope (HST).

• The median \tau$_\mathsf{eff,HeII}$ values increase gradually from 1.95 at z=2.7 to 5.17 at z=3.4, but with a strong sightline-to-sightline variance.
• Many $\simeq$35 comoving Mpc regions of the z>3 intergalactic medium (IGM) remain transmissive (\tau$_\mathsf{eff,HeII}$<4), and the gradual trend with redshift appears consistent with density evolution of a fully reionized IGM.
• These modest optical depths imply average HeII fractions of x$_\mathsf{HeII}$<0.01 and HeII ionizing photon mean free paths of $\simeq$50 comoving Mpc at z$\simeq$3.4, thus requiring that a substantial volume of the helium in the Universe was already doubly ionized at early times; this stands in conflict with current models of HeII reionization driven by luminous quasars.