2.5.2. Nuclear Activity: Feeding the Monster
Simkin, Su and Schwarz (1980, hereafter S3) suggest that Seyfert galaxies contain morphological features which tend to transport disk gas toward the nucleus. This is the effect of any structures which produce large-scale non-axisymmetric distortions in the potential field (section 5.4). The most obvious such feature is a bar, and many Seyferts are found to be barred (Adams 1977). However, we know from theoretical calculations (section 5.4) that large-scale oval distortions with axial ratios of ~ 0.8 are as effective at transporting gas as more elongated bars which contribute less of the mass than a typical oval. A large number of Seyferts are, in fact, oval (S3; see also Adams 1977, who noted the prevalence of outer rings, which were seen above to be characteristic of ovals). For example, the well-known Seyferts NGC 1068 (Hubble Atlas) and NGC 4151 (Fig. 4) contain prototypical oval disks. Another example is NGC 1566 (Fig. 2). Finally, strong interactions with a companion can have a similar effect; examples of interacting Seyferts are also common and are illustrated in Adams (1977). In agreement with S3, those Seyferts for which I was able to find good published photographs all showed at least one of the above features which plausibly could drive gas toward the nucleus. However, there are still many Seyfert galaxies without good published photographs. A more complete survey would be valuable.
Simkin, Su and Schwarz plausibly speculate that the disk gas is "feeding the monster" (Gunn 1977a) in the nucleus, thereby powering the activity. The rarity of elliptical Seyferts (van den Bergh 1975b, Adams 1977) may result from the fact that ellipticals lack gas. Gas transport by global asymmetries will, of course, not operate at radii r << 1 kpc, where the distortions are not felt. At such small galactic radii (but large radii from any compact object at the center) some other process, such as that discussed by Gunn (1977a), must take over. Morphology once again provides strong hints for the quantitative work needed to establish whether the above gas transport process helps to fuel nuclear activity.