The History and Morphology of Helium Reionization

TL;DR

This paper examines the evolution and morphology of helium reionization driven by quasars around redshift 3, highlighting the impact of source rarity and clustering on the growth of ionized regions.

Contribution

It provides a detailed analysis of helium reionization stages, emphasizing the role of quasar distribution and clustering in shaping ionized gas morphology.

Findings

Early ionized bubbles had radii of 15-20 Mpc due to stochastic quasar distribution.

Large-scale quasar clustering influenced bubble sizes when ionized fraction exceeded 50%.

Recombinations limited bubble growth to about 35-40 Mpc at high ionized fractions.

Abstract

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_\star), random fluctuations in the quasar population determined the morphology of ionized gas when the global ionized fraction x_i was small, with the typical radius R_c of a HeIII bubble ~15-20 comoving Mpc. Only when x_i>0.5 did the large-scale clustering of the quasars drive the characteristic size of ionized regions above this value. Still later, when x_i>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 R_c~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.