The hierarchical paradigm holds major mergers responsible for triggering starbursts and for the transport of gas to nuclear regions leading to AGN activity, with AGN feedback potentially heating and clearing away remaining gas from the host and terminating star formation. Incidentally, major mergers also lead to the formation of spheroidal galaxies, which will need a Gyr or more to settle onto the red sequence of non-star-forming, passively evolving galaxies. This picture is broadly consistent with the observations of AGN host galaxies.
Observations of a relationship between quiescent black-hole masses and
the velocity dispersion of the hosting galaxy bulge
(Ferrarese & Merritt
2000;
Gebhardt et al. 2000)
have suggested a co-evolution of galaxies and AGN. This relation can be
further developed into a relation between
MBH and stellar mass of the bulge. However, recently,
Treu, Malkan & Blandford
(2004)
found the MBH-
relation at z = 0.37 offset from the one at z
= 0. If this result is confirmed, it implies that AGN were active and
black holes have grown before the stellar bulges reached their final
mass. On a currently more speculative side, high-resolution VLA
observation made by
Walter et al. (2004)
support this view, claiming
that the host galaxy of the highest-redshift QSO known, SDSS J1148+5251
at z = 6.42, consists mostly of molecular gas, with very little
room left for stellar mass, effectively ruling out the presence of a
1012 Msol stellar bulge required by the local
MBH-
relation.