  While supermassive black holes (SMBHs) play an important role in galaxy and
cluster evolution, at present they can only be included in large-scale
cosmological simulation via subgrid techniques. However, these subgrid models
have not been studied in a systematic fashion. Using a newly-developed fast,
parallel spherical overdensity halo finder built into the simulation code
FLASH, we perform a suite of dark matter-only cosmological simulations to study
the effects of subgrid model choice on relations between SMBH mass and dark
matter halo mass and velocity dispersion. We examine three aspects of SMBH
subgrid models: the choice of initial black hole seed mass, the test for
merging two black holes, and the frequency of applying the subgrid model. We
also examine the role that merging can play in determining the relations,
ignoring the complicating effects of SMBH-driven accretion and feedback. We
find that the choice of subgrid model can dramatically affect the black hole
merger rate, the cosmic SMBH mass density, and the low-redshift relations to
halo properties. We also find that it is possible to reproduce observations of
the low-redshift relations without accretion and feedback, depending on the
choice of subgrid model.
