(Abridged) Using ab initio cosmological Eulerian adaptive mesh refinement
radiation hydrodynamical calculations, we discuss how very massive stars start
the process of cosmological reionization. The models include non-equilibrium
primordial gas chemistry and cooling processes and accurate radiation transport
in the Case B approximation using adaptively ray traced photon packages,
retaining the time derivative in the transport equation. Supernova feedback is
modeled by thermal explosions triggered at parsec scales. All calculations
resolve the local Jeans length by at least 16 grid cells at all times and as
such cover a spatial dynamic range of ~10^6. These first sources of
reionization are highly intermittent and anisotropic and first photoionize the
small scales voids surrounding the halos they form in, rather than the dense
filaments they are embedded in. As the merging objects form larger, dwarf sized
galaxies, the escape fraction of UV radiation decreases and the HII regions
only break out on some sides of the galaxies making them even more anisotropic.
In three cases, SN blast waves induce star formation in overdense regions that
were formed earlier from ionization front instabilities. These stars form tens
of parsecs away from the center of their parent DM halo. Approximately 5
ionizing photons are needed per sustained ionization when star formation in
10^6 M_sun halos are dominant in the calculation. As the halos become larger
than ~10^7 M_sun, the ionizing photon escape fraction decreases, which in turn
increases the number of photons per ionization to 15-50, in calculations with
stellar feedback only. Supernova feedback in these more massive halos creates a
more diffuse medium, allowing the stellar radiation to escape more easily and
maintaining the ratio of 5 ionizing photons per sustained ionization.Comment: 16 pages, 15 figures, accepted to ApJ. Final version. High resolution
images and movies available at
http://www.slac.stanford.edu/~jwise/research/Reionizatio
