Photo-heating and the fate of hard photons during the reionisation of HeII by quasars

TL;DR

This paper investigates the impact of quasar photo-heating during HeII reionisation on the intergalactic medium's thermal evolution, concluding that rapid, strong heating is unlikely across the entire IGM due to spectral and environmental constraints.

Contribution

It combines analytic and numerical methods to assess the limits of photo-heating during HeII reionisation, highlighting the role of quasar spectra and dense clumps in heating efficiency.

Findings

Rapid, strong heating (^4 K) is difficult unless quasar spectra are very hard.

Filtering of radiation increases energy per ionisation but reduces ionisation rate, limiting heating.

Most of the IGM is reionised by softer photons, resulting in modest heating (^4 K).

Abstract

We use a combination of analytic and numerical arguments to consider the impact of quasar photo-heating during HeII reionisation on the thermal evolution of the intergalactic medium (IGM). We demonstrate that rapid (\Delta z< 0.1-0.2), strong (\Delta T > 10^4 K) photo-heating is difficult to achieve across the entire IGM unless quasar spectra are significantly harder than implied by current observational constraints. Although filtering of intrinsic quasar radiation through dense regions in the IGM does increase the mean excess energy per HeII photo-ionisation, it also weakens the radiation intensity and lowers the photo-ionisation rate, preventing rapid heating over time intervals shorter than the local photo-ionisation timescale. Moreover, the hard photons responsible for the strongest heating are more likely to deposit their energy inside dense clumps. The abundance of such clumps is, however, uncertain and model-dependent, leading to a fairly large uncertainty in the photo-heating rates. Nevertheless, although some of the IGM may be exposed to a hardened and weakened ionising background for long periods, most of the IGM must instead be reionised by the more abundant, softer photons and with accordingly modest heating rates (\Delta T < 10^4 K). The repeated ionisation of fossil quasar HeIII regions does not increase the net heating because the recombination times in these regions typically exceed the IGM cooling times and the average time lag between successive rounds of quasar activity. Detailed line-of-sight radiative transfer simulations confirm these expectations and predict a rich thermal structure in the IGM during HeII reionisation. [Abridged]