The imprint of inhomogeneous He II reionization on the H I and He II Lyα forest

We use a set of adaptive mesh refinement hydrodynamic simulations post-processed with the radiative-transfer code RADAMESH to study how inhomogeneous He ii reionization affects the intergalactic medium (IGM). We propagate radiation from active galactic nuclei (AGNs) characterized by a finite lifetime and located within massive dark matter haloes. We consider two scenarios for the time evolution of the ionizing sources. In all cases, we find that He ii reionization takes place in a very inhomogeneous fashion, through the production of well-separated bubbles of the ionized phase that subsequently percolate. Overall, the reionization process is extended in time and lasts for a redshift interval delta z greater than or similar to 1. At fixed gas density, the temperature distribution is bimodal during the early phases of He ii reionization and cannot be described by a simple effective equation of state. When He ii reionization is complete, the IGM is characterized by a polytropic equation of state with index gamma similar or equal to 1.20. This relation is appreciably flatter than at the onset of the reionization process (gamma = 1.56) and also presents a much wider dispersion around the mean. We extract H i and He ii Ly alpha absorption spectra from the simulations and fit Voigt profiles to them. Our results are fully consistent with the observed evolution in the mean transmission and the flux power spectrum. We find that the regions where helium is doubly ionized are characterized by different probability density functions of the curvature and of the Doppler b parameters of the H i Ly alpha forest as a consequence of the bimodal temperature distribution during the early phases of He ii reionization. The column-density ratio in He ii and H i shows a strong spatial variability. Its probability density function rapidly evolves with time reflecting the increasing volume fraction in which ionizing radiation is harder due to the AGN contribution. Finally, we show that the number density of the flux-transmission windows per unit redshift and the mean size of the dark gaps in the He ii spectra have the potential to distinguish between different reionization scenarios.