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

Gabor Worseck

AIP Potsdam

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 2.74 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.