The History and Morphology of Helium Reionization

A variety of observations now indicate that intergalactic helium was fully ionized by z~3. The most recent measurements of the high-redshift quasar luminosity function imply that these sources had produced ~2. 5 ionizing photons per helium atom by that time, consistent with a picture in which the known quasar population drives HeII reionization. Here we describe the distribution of ionized and neutral helium gas during this era. Because the sources were rare and bright (with the photon budget dominated by quasars with luminosities L>L_), random fluctuations in the quasar population determined the morphology of ionized gas when the global ionized fraction xᵢ was small, with the typical radius Rc of a HeIII bubble ~15-20 comoving Mpc. Only when xᵢ>0. 5 did the large-scale clustering of the quasars drive the characteristic size of ionized regions above this value. Still later, when xᵢ>0. 75, most ionizing photons were consumed by dense, recombining systems before they reached the edge of their source's ionized surroundings, halting the bubble growth when Rc~35-40 Mpc. These phases are qualitatively similar to those in hydrogen reionization, but the rarity of the sources makes the early stochastic phase much more important. Moreover, the well-known characteristics of the z=3 intergalactic medium allow a much more robust description of the late phase in which recombinations dominate.