Spatial fluctuations of the intergalactic temperature-density relation after hydrogen reionization

TL;DR

This study models the post-reionization intergalactic medium's temperature-density relation, revealing significant spatial fluctuations that impact Ly-alpha forest observations and provide insights into reionization history.

Contribution

It introduces cosmological radiative transfer simulations with realistic, inhomogeneous reionization, calibrated with Ly-alpha data, to analyze spatial fluctuations in the IGM's thermal state.

Findings

Spatial fluctuations in TDR are significant after reionization.

Recently ionized regions are hotter and have flatter TDR.

Fluctuations affect Ly-alpha opacity statistics at z ~ 4-6.

Abstract

The thermal state of the post-reionization IGM is sensitive to the timing of reionization and the nature of the ionizing sources. We have modelled here the thermal state of the IGM in cosmological radiative transfer simulations of a realistic, extended, spatially inhomogeneous hydrogen reionization process, carefully calibrated with Ly-alpha forest data. We compare these with cosmological simulations run using a spatially homogeneous ionizing background. The simulations with a realistic growth of ionized regions and a realistic spread in reionization redshifts show, as expected, significant spatial fluctuations in the temperature-density relation (TDR) of the post-reionization IGM. The most recently ionized regions are hottest and exhibit a flatter TDR. In simulations consistent with the average TDR inferred from Ly-alpha forest data, these spatial fluctuations have a moderate but noticeable effect on the statistical properties of the Ly-alpha opacity of the IGM at z ~ 4-6. This should be taken into account in accurate measurements of the thermal properties of the IGM and the free-streaming of dark matter from Ly-alpha forest data in this redshift range. The spatial variations of the TDR predicted by our simulations are, however, smaller by about a factor two than would be necessary to explain the observed large spatial opacity fluctuations on large (> 50 comoving Mpc/h) scales at z > 5.5.