Constraints on the High-Redshift Quasar Population from Ongoing Helium Reionization at z~4

Akin to neutral hydrogen, intergalactic singly ionized helium can be probed by Lyman alpha forest spectroscopy of quasars. The advent of GALEX and HST/COS have revolutionized our view of HeII reionization, the final major phase transition of the intergalactic medium. Efficient surveys pioneered by my team densely sample the strongly fluctuating HeII absorption at 2.7<z<3, evoking a picture of overlapping HeIII bubbles around quasars at the end of HeII reionization at z~2.7. Unexpectedly, the seven z>3.5 sightlines observed to date show several Lyman alpha and beta transmission spikes that suggest HeII reionization was well underway by z=4. Their occurrence is in conflict with predictions from early radiative transfer simulations of a HeII reionization driven by bright quasars, and suggests a high space density of faint quasars at z>4 that may have a non-negligible contribution to hydrogen reionization. Moreover, due to the long HeII photoionization timescale in the intergalactic medium, the highly ionized HeII proximity zones of our discovered quasars enable the first precise measurements of the duration of individual quasar accretion episodes of up to ~30 Myr.