Photo-heating and the fate of hard photons during the reionisation of HeII by quasars
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 10^4K) photo-heating is extremely 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 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, which cool rapidly and are furthermore invisible to most observational probes of the IGM temperature. As a result, although some of the IGM may be exposed to a hardened and weakened ionising background for long periods, most of the IGM is reionised by the more abundant, softer photons and the net heating is accordingly small (\Delta T_HeII < 5,000-10,000K). However, localised patches of much higher temperatures are still likely. 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. Detailed line-of-sight radiative transfer simulations confirm these expectations and predict a rich thermal structure in the IGM during HeII reionisation. The resulting complex relationship between temperature and density may help resolve discrepancies between optically thin simulations of the Lya forest and recent observations.Keywords
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