  We explore the growth of super-massive black holes and host galaxy bulges in
the galaxy population using the Millennium Run LCDM simulation coupled with a
model of galaxy formation. We find that, if galaxy mergers are the primary
drivers for both bulge and black hole growth, then in the simplest picture one
should expect the mBH-mbulge relation to evolve with redshift, with a larger
black hole mass associated with a given bulge mass at earlier times relative to
the present day. This result is independent of an evolving cold gas fraction in
the galaxy population. The evolution arises from the disruption of galactic
disks during mergers that make a larger fractional mass contribution to bulges
at low redshift than at earlier epochs. There is no comparable growth mode for
the black hole population. Thus, this effect produces evolution in the
mBH-mbulge relation that is driven by bulge mass growth and not by black holes.

Astrophysics