Tracing the Sources of Reionization in Cosmological Radiation
  Hydrodynamics Simulations
  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.
