Evolution in the black hole mass-bulge mass relation: a theoretical perspective

We explore the growth of supermassive black holes and host galaxy bulges in the galaxy population using the Millennium Run Lambda cold dark matter (Lambda CDM) 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 m(BH)-m(bulge) 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 discs 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 m(BH)-m(bulge) relation that is driven by bulge mass growth and not by black holes.