The Evolution of the HeII-Ionizing Background at Redshifts 2.3<z<3.8 Inferred from a Statistical Sample of 24 HST/COS HeII Ly$\alpha$ Absorption Spectra

Understand

We present measurements of the large-scale (~40 comoving Mpc) effective optical depth of HeII Ly$\alpha$ absorption, $\tau_{\rm eff}$, at 2.54<z<3.86 toward 16 HeII-transparent quasars observed with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST), to characterize the ionization state of helium in the intergalactic medium (IGM).

• We provide the first statistical sample of $\tau_{\rm eff}$ measurements in six signal-to-noise ratio >3 HeII sightlines at z>3.5, and study the redshift evolution and sightline-to-sightline variance of $\tau_{\rm eff}$ in 24 HeII sightlines.
• We confirm an increase of the median $\tau_{\rm eff}$ from ~2 at z=2.7 to >5 at z>3, and a scatter in $\tau_{\rm eff}$ that increases with redshift.
• The z>3.5 HeII absorption is predominantly saturated, but isolated narrow ($\Delta v<650$ km/s) transmission spikes indicate patches of reionized helium.