HeII Reionization and its Effect on the IGM

TL;DR

This study uses large-scale simulations to analyze how HeII reionization by quasars heats the intergalactic medium, affecting temperature, fluctuations, and observable signatures, with implications for understanding cosmic reionization history.

Contribution

The paper presents detailed simulations of HeII reionization, revealing its heating effects, temperature fluctuations, and impact on observable IGM properties, which were not previously quantified at this scale.

Findings

HeII reionization heats the IGM by up to 25,000 K.

Temperature fluctuations occur on ~50 Mpc scales.

HeII reionization does not invert the temperature-density relation.

Abstract

Observations of the intergalactic medium (IGM) suggest that quasars reionize HeII in the IGM at z ~ 3. We have run a set of 190 and 430 comoving Mpc simulations of HeII being reionized by quasars to develop an understanding of the nature of HeII reionization and its potential impact on observables. We find that HeII reionization heats regions in the IGM by as much as 25,000 K above the temperature that is expected otherwise, with the volume-averaged temperature increasing by ~ 12,000 K and with large temperature fluctuations on ~ 50 Mpc scales. Much of the heating occurs far from QSOs by hard photons. We find a temperature-density equation of state of gamma -1 ~ 0.3 during HeII reionization, but with a wide dispersion in this relation having sigma ~ 10^4 K. HeII reionization by the observed population of quasars cannot produce an inverted relation (gamma - 1 < 0). Our simulations are consistent with the observed evolution in the mean transmission of the HeII Ly-alpha forest. We argue that the heat input due to HeII reionization is unable to cause the observed depression at z = 3.2 in the HI Ly-alpha forest opacity as has been suggested. We investigate how uncertainties in the properties of QSOs and of HeII Lyman-limit systems influence our predictions.