8. RINGS AND NUCLEAR ACTIVITY
Several studies have noted a preponderance of ring, pseudoring, and bar features in Seyfert galaxies. Adams (1977) noted that Seyferts with annular main bodies form a natural group. Later studies by Simkin et al. (1980), Su & Simkin (1980), and Arsenault (1989) elucidated this observation further. Simkin et al. (1980) used a sample of nearby Seyferts to determine that Seyferts have a preference for barred and weakly-barred systems with inner rings or pseudorings. They suggested that the nuclear activity is connected to the way a bar-like perturbation can funnel gas into the central region, causing a pile-up near the inner Lindblad resonance (ILR). As the central density increases, an ILR may grow more effective, maintaining the process. The idea has been supported by Shlosman et al. (1989), Friedli & Benz (1993), and Heller & Shlosman (1994). We discuss it further in section 12.4.
Su and Simkin (1980) found a correlation between disk morphology in Seyfert 1 galaxies and the full width at zero intensity (FWZI) of the Balmer emission lines. Objects ranging from pure spirals (S), inner rings with outer spirals (RS), inner rings and outer rings (RR), inner disks and outer rings (DR), and amorphous structure (A) showed a general increase in FWZI. This was interpreted as due to the evolution of a system with a bar or oval, the gas distribution developing initially into spirals, then torqued into rings, and ending with the perturbation eventually dissolving leading to the diffusion of the rings.
Arsenault (1989) tried to elucidate the correspondence between nuclear activity and the presence of bars and rings by selecting samples of starburst and Seyfert galaxies and comparing the frequency of bars and rings in such systems with that in a control sample of galaxies. He concluded that bars and rings are more prevalent in AGN and starburst galaxies than in ``normal'' (non-AGN and non-starburst) galaxies. Moles et al. (1995) examined the same question using Seyfert and LINER galaxies from the catalog of Véron-Cetty & Véron (1991) and morphological data from RC3. Of 186 active galaxies having sufficient morphological information, 95% were found to have rings and bars. The rings and the activity were suggested to be effects that accompany the non-axisymmetrical perturbations. The debate continues, however. Ho et al. (1996) carried out a spectroscopic survey of nearly 500 nearby galaxies, and concluded that while bars contribute significantly to circumnuclear star formation, they do not necessarily have an obvious influence on nuclear activity.