  We use a suite of high-resolution N-body simulations to study the properties,
abundance and clustering of high mass halos at high redshift, including their
mass assembly histories and mergers. We find that the analytic form which best
fits the abundance of halos depends sensitively on the assumed definition of
halo mass, with common definitions of halo mass differing by a factor of two
for these low concentration, massive halos. A significant number of massive
halos are undergoing rapid mass accretion, with major merger activity being
common. We compare the mergers and mass accretion histories to the extended
Press-Schechter formalism.
  We consider how major merger induced star formation or black hole accretion
may change the distribution of photon production from collapsed halos, and
hence reionization, using some simplified examples. In all of these, the photon
distribution for a halo of a given mass acquires a large scatter. If rare, high
mass halos contribute significantly to the photon production rates, the scatter
in photon production rate can translate into additional scatter in the sizes of
ionized bubbles.
