  It is shown that, if gas accretion via a disk onto the central supermassive
black hole is efficient only for surface density Sigma >= 10 g/cm^2, the black
hole mass - galactic bulge velocity dispersion relation (Tremaine et al 2002)
is naturally borne out, so is the modest dispersion in that relation, in the
context of hierarchical structure formation theory. This relation is not
expected to evolve with redshift in this model.
