Growth and anisotropy of ionization fronts near high redshift quasars in
  the MassiveBlack simulation
  We use radiative transfer to study the growth of ionized regions around the
brightest, z=8 quasars in a large cosmological hydrodynamic simulation that
includes black hole growth and feedback (the MassiveBlack simulation). We find
that in the presence of the quasar s the comoving HII bubble radii reach 10
Mpc/h after 20 My while with the stellar component alone the HII bubbles are
smaller by at least an order of magnitude. Our calculations show that several
features are not captured within an analytical growth model of Stromgren
spheres. The X-ray photons from hard quasar spectra drive a smooth transition
from fully neutral to partially neutral in the ionization front. However the
transition from partially neutral to fully ionized is significantly more
complex. We measure the distance to the edge of bubbles as a function of angle
and use the standard deviation of these distances as a diagnostic of the
isotropy of ionized regions. We find that the overlapping of nearby ionized
regions from clustered halos not only increases the anisotropy, but also is the
main mechanism which allows the outer radius to grow. We therefore predict that
quasar ionized bubbles at this early stage in the reionization process should
be both significantly larger and more irregularly shaped than bubbles around
star forming galaxies. Before the star formation rate increases and the
Universe fully reionizes, quasar bubbles will form the most striking and
recognizable features in 21cm maps.
