The impact of helium reionization on the structure of the intergalactic medium

We examine the impact of helium reionization on the structure of the intergalactic medium. We model the reionization using a radiative transfer (RT) code coupled to the combined gravity hydrodynamics code enzo. Neutral hydrogen and helium are initially ionized by a starburst spectrum, which is allowed to gradually evolve into a power-law spectrum over the redshift interval 3.0 < z < 4.0. The temperature–density relation of the gas is found to fan out and flatten following He ii reionization, with an inversion for high overdensities of ρ/〈ρ〉 > 5. Peculiar velocities of up to 10 km s−1 are induced by the increased pressure, with the gas density field distorted over large coherent regions by 10–20 per cent, and the dark matter by levels of 1 per cent. The photoionization-induced flows may thus distort the matter power spectrum at comoving wavenumbers k > 0.5 h Mpc−1 by a few per cent by z= 2. Absorption spectra for H i and He ii are drawn from the simulations, and absorption lines are fitted to the spectra. The increased temperature following He ii reionization reduces the line centre optical depths, resulting in an enhancement in the fraction of very low optical depth pixels and an overall distortion in the pixel flux distribution compared with He ii reionization in the optically thin limit. A median Doppler parameter of 35 km s−1 is obtained for the H i absorption systems at z= 3. Dividing into subsamples optically thick and optically thin at line centre reveals that the optically thick systems undergo only mild evolution while the optically thin systems evolve rapidly following He ii reionization. A comparison between He ii and H i absorption features shows a broad distribution in the He ii and H i column density ratio, although much of the width is an artefact of the deblending of the absorption features by the line fitting. The distribution of the ratio of He ii and H i Doppler parameters peaks midway between the thermally broadened and velocity-broadened limits. A comparison with approximate simulation methods based on either a pseudo-hydrodynamical scheme or a non-RT hydrodynamical simulation with boosted He ii heating rate shows moderately good agreement in the absorption line properties, but not to the precision to which they may be measured, and not over the full redshift range for which the high-redshift Lyα forest is observed.