  We investigate the formation of the first massive black holes in high
redshift galaxies, with the goal of providing insights to which galaxies do or
do not host massive black holes. We adopt a novel approach to forming seed
black holes in galaxy halos in cosmological SPH+N-body simulations. The
formation of massive black hole seeds is dictated directly by the local gas
density, temperature, and metallicity, and motivated by physical models of
massive black hole formation. We explore seed black hole populations as a
function of halo mass and redshift, and examine how varying the efficiency of
massive black hole seed formation affects the relationship between black holes
and their hosts. Seed black holes tend to form in halos with mass between 10^7
and 10^9 Msun, and the formation rate is suppressed around z = 5 due to the
diffusion of metals throughout the intergalactic medium. We find that the time
of massive black hole formation and the occupation fraction of black holes are
a function of the host halo mass. By z = 5, halos with mass M_halo > 3 x 10^9
Msun host massive black holes regardless of the efficiency of seed formation,
while the occupation fraction for smaller halos increases with black hole
formation efficiency. Our simulations explain why massive black holes are found
in some bulgeless and dwarf galaxies, but we also predict that their occurrence
becomes rarer and rarer in low-mass systems.
