Three-Dimensional Modelling of Ionized Gas. I. Did very massive stars of
  different metallicities drive the second cosmic reionization?
  The first generation of stars that formed directly from the primordial gas
played a crucial role in the early phase of the reionization of the universe.
Because of the short lifetimes of these stars the metals produced in their
cores were quickly returned to the environment, from which early PopII stars
with a different initial mass function and different SEDs were formed, already
much earlier than the time at which the universe became completely reionized at
a redshift of z~6. Using a state-of-the-art model atmosphere code we calculate
realistic SEDs of very massive stars (VMSs) of different metallicities to serve
as input for the 3-dimensional multi-frequency radiative transfer code we have
developed to simulate the temporal evolution of the ionization of the
inhomogeneous interstellar and intergalactic medium, using multiple stellar
clusters as sources of ionizing radiation. Our tool handles distributions of
numerous radiative sources characterized by high resolution synthetic SEDs, and
yields occupation numbers of the required energy levels of the most important
elements which are treated in NLTE consistently with the 3d radiative transfer.
We further demonstrate that the increasing metallicity of the radiative sources
in the transition from PopIII stars to PopII stars has a strong impact on the
hardness of the emitted spectrum, and hence on the reionization history of
helium. A top-heavy stellar mass distribution characterized by VMSs forming in
chemically evolved clusters of high core mass density may not only provide the
progenitors of intermediate-mass and supermassive black holes (SMBHs), but also
play an important role for the reionization of HeII. The number of VMSs
required to reionize HeII by a redshift of z~2.5 is astonishingly close to the
number of VMSs required to explain galactic SMBHs if one assumes that these
have been formed by mergers of smaller black holes.
