Hydrogen Reionization in the Illustris universe
  Hydrodynamical simulations of galaxy formation such as the Illustris
simulations have progressed to a state where they approximately reproduce the
observed stellar mass function from high to low redshift. This in principle
allows self-consistent models of reionization that exploit the accurate
representation of the diffuse gas distribution together with the realistic
growth of galaxies provided by these simulations, within a representative
cosmological volume. In this work, we apply and compare two radiative transfer
algorithms implemented in a GPU-accelerated code to the $106.5\,{\rm Mpc}$ wide
volume of Illustris in postprocessing in order to investigate the reionization
transition predicted by this model. We find that the first generation of
galaxies formed by Illustris is just about able to reionize the universe by
redshift $z\sim 7$, provided quite optimistic assumptions about the escape
fraction and the resolution limitations are made. Our most optimistic model
finds an optical depth of $\tau\simeq 0.065$, which is in very good agreement
with recent Planck 2015 determinations. Furthermore, we show that moment-based
approaches for radiative transfer with the M1 closure give broadly consistent
results with our angular-resolved radiative transfer scheme. In our favoured
fiducial model, 20\% of the hydrogen is reionized by redshift $z=9.20$, and
this rapidly climbs to 80\% by redshift $z=6.92$. It then takes until $z=6.24$
before 99\% of the hydrogen is ionized. On average, reionization proceeds
`inside-out' in our models, with a size distribution of reionized bubbles that
progressively features regions of ever larger size while the abundance of small
bubbles stays fairly constant.
