# Dark-Ages Reionisation & Galaxy Formation Simulation XVI: The Thermal Memory of Reionisation

## TL;DR

This study uses thermal statistics of the intergalactic medium to constrain the epoch of reionisation, revealing how temperature evolution retains memory of reionisation timing and spectral properties.

## Contribution

It introduces a method to constrain reionisation timing and spectral slope using thermal history and temperature-density relations from galaxy formation models.

## Key findings

- Reionisation likely completed at redshift ~6.8.
- Thermal memory diminishes as gas cools, forming a power-law temperature-density relation.
- Future observations can break degeneracies between reionisation timing and spectral properties.

## Abstract

Intergalactic medium temperature is a powerful probe of the epoch of reionisation, as information is retained long after reionisation itself. However, mean temperatures are highly degenerate with the timing of reionisation, with the amount heat injected during the epoch, and with the subsequent cooling rates. We post-process a suite of semi-analytic galaxy formation models to characterise how different thermal statistics of the intergalactic medium can be used to constrain reionisation. Temperature is highly correlated with redshift of reionisation for a period of time after the gas is heated. However as the gas cools, thermal memory of reionisation is lost, and a power-law temperature-density relation is formed, $T = T_0(1+\delta)^{1-\gamma}$ with $\gamma \approx 1.5$. Constraining our model against observations of electron optical depth and temperature at mean density, we find that reionisation likely finished at $z_{\rm{reion}} = 6.8 ^{+ 0.5} _{-0.8}$ with a soft spectral slope of $\alpha = 2.8 ^{+ 1.2} _{-1.0}$. By restricting spectral slope to the range $[0.5,2.5]$ motivated by population II synthesis models, reionisation timing is further constrained to $z_{\rm{reion}} = 6.9 ^{+ 0.4} _{-0.5}$. We find that, in the future, the degeneracies between reionisation timing and background spectrum can be broken using the scatter in temperatures and integrated thermal history.