Tracing the sources of reionization in cosmological radiation hydrodynamics simulations

Abstract

We use the photon flux and absorption tracer algorithm presented in Katz et al.~2018, to characterise the contribution of haloes of different mass and stars of different age and metallicity to the reionization of the Universe. We employ a suite of cosmological multifrequency radiation hydrodynamics AMR simulations that are carefully calibrated to reproduce a realistic reionization history and galaxy properties at $z \geq 6$. In our simulations, haloes with mass $10^9{\rm M_{\odot}}h^{-1}<M<10^{10}{\rm M_{\odot}}h^{-1}$, stars with metallicity $10^{-3}Z_{\odot}<Z<10^{-1.5}Z_{\odot}$, and stars with age $3\,{\rm Myr} < t < 10 \, {\rm Myr}$ dominate reionization by both mass and volume. We show that the sources that reionize most of the volume of the Universe by $z=6$ are not necessarily the same sources that dominate the meta-galactic UV background at the same redshift. We further show that in our simulations, the contribution of each type of source to reionization is not uniform across different gas phases. The IGM, CGM, filaments, ISM, and rarefied supernova heated gas have all been photoionized by different classes of sources. Collisional ionisation contributes at both the lowest and highest densities. In the early stages of the formation of individual HII bubbles, reionization proceeds with the formation of concentric shells of gas ionised by different classes of sources, leading to large temperature variations as a function of galacto-centric radius. The temperature structure of individual HII bubbles may thus give insight into the star formation history of the galaxies acting as the first ionising sources. Our explorative simulations highlight how the complex nature of reionization can be better understood by using our photon tracer algorithm.