  Nuclear activity in galaxies is closely connected to galactic mergers and
supermassive black holes (SBH). Galactic mergers perturb substantially the
dynamics of gas and stellar population in the merging galaxies, and they are
expected to lead to formation of supermassive binary black holes (BBH) in the
center of mass of the galaxies merged. A scheme is proposed here that connects
the peak magnitude of the nuclear activity with evolution of a BBH system. The
scheme predicts correctly the relative fractions of different types of active
galactic nuclei (AGN) and explains the connection between the galactic type and
the strength of the nuclear activity. It shows that most powerful AGN should
result from mergers with small mass ratios, while weaker activity is produced
in unequal mergers. The scheme explains also the observed lack of galaxies with
two active nuclei, which is attributed to effective disruption of accretion
disks around the secondary in BBH systems with masses of the primary smaller
than ~10^10 solar masses.
