The hydrodynamic response of small-scale structure to reionization drives large IGM temperature fluctuations that persist to z = 4

The thermal history and structure of the intergalactic medium (IGM) at z 4 is a key boundary condition for reionization, which has been measured using the Ly forest of high-redshift quasars. It is also a key input for studies that use the forest to constrain the particle masses of alternative dark matter candidates. Most such inferences rely on simulations that lack the spatial resolution to fully resolve the hydrodynamic response of IGM filaments and minihalos caused by HI reionization heating. In this letter, we use high-resolution hydrodynamic+radiative transfer simulations to study the impact of these on the IGM thermal structure. We find that the adiabatic heating and cooling driven by the expansion of initially cold gas filaments and minihalos drives significant temperature fluctuations on small scales. These likely persist in much of the IGM until at least z = 4. Capturing this effect requires resolving the characteristic clumping scale of cold, pre-ionized gas, which demands spatial resolutions of at least 2 h^-1kpc. Pre-heating of the IGM by X-Ray sources can slightly reduce the effect. Our preliminary estimate of the effect on the Ly forest finds that, at (k / km^{-1 s}) = -1. 0, the forest flux power (at fixed mean flux) can increase by 10-20\% when going from 8 and 2 h^-1kpc resolution at z = 4-5 for gas ionized at z < 7. These findings motivate a more careful analysis of how temperature fluctuations driven by pressure smoothing from reionization affect the Ly forest.