On the Role of Minor Galaxy Mergers in the Formation of Active Galactic Nuclei

The large scale (~ 100 kpc) environments of Seyfert galaxies are notsignificantly different from those of non-Seyfert galaxies. In the context ofthe interaction model of the formation of active galactic nuclei (AGN), it hasbeen proposed that AGN form via "minor mergers" of large disk galaxies withsmaller companions. We test this hypothesis by comparing the nuclear spectra of105 bright nearby galaxies with measurements of their R or r band morphologicalasymmetries at three successive radii. We find no significant differences inthese asymmetries between the 13 Seyfert galaxies in the sample and galaxieshaving other nuclear spectral types (absorption, H II-region like, LINER), noris there strong qualitative evidence that such mergers have occured among anyof the Seyferts or LINERs. Thus either any minor mergers began > 1 Gyr ago andare essentially complete, or they did not occur at all, and AGN formindependently of any type of interaction. Support for the latter interpretationis provided by the growing evidence that supermassive black holes exist in thecores of most elliptical and early-type spiral galaxies, which in turn suggeststhat nuclear activity represents a normal phase in the evolution of the bulgesof massive galaxies. Galaxy mergers may increase the luminosity of Seyfertnuclei to the level of QSOs, which could explain why the latter objects appearto be found in rich environments and in interacting systems.Comment: 13 pages, 2 figures, to appear in Astrophysical Journal Letter