|Annu. Rev. Astron. Astrophys. 1982. 20:
Copyright © 1982 by . All rights reserved
3.4. Bars, Rings, and Disk Anomalies
The significance of morphological anomalies in active galaxies is another subject riddled with vague impression and controversy. Among active ellipticals in noninteracting systems there are few reports of structural peculiarities in the stellar distribution. For this reason, our attention turns to disk galaxies with activity, and of these the population studied most intensively for morphological anomalies is the Seyferts. Adams (1977) studied the morphologies of some eighty Seyferts and reviewed the earlier results and conclusions. More recently Heckman (1978), Su & Simkin et al. (1980 - hereafter SS), and, using extensive new observations, Simkin et al. (1980 - hereafter SSS) have reconsidered differences in disk morphologies between Seyfert and comparable normal galaxies.
It now appears as if there is no statistically significant excess of bars in galaxies having LINERs, flat-spectrum compact radio sources, or Seyfert nuclei (Heckman 1980c, SSS). Heckman further finds that star formation is enhanced in the near-nuclear regions of barred spirals, and this may explain Hummel's (1980) result that the total nuclear (steep-spectrum) radio emission of barred spirals exceeds that of unbarred systems by a factor of two (see Condon et al. 1982). The absence of enhanced nuclear activity in barred systems comes as a surprise; models (e.g. Sanders & Huntley 1976, Sancisi et al. 1979, Huntley 1980, Huntley et al. 1978) and observations (e.g. Huntley 1978, Peterson & Huntley 1980, Peterson et al. 1978) indicate that gas can flow into or near the nuclei of such galaxies. Perhaps the explanation is that the ready availability of food for nuclear activity is no particular problem for spiral galaxies. Alternatively, the flow rate of gas through the nucleus in a barred spiral may be much less than predicted (as recent work by van Albada & Roberts (1981) suggests), or the food may be in an "indigestible" form.
SSS have shown that stellar rings or ring-like structure, unlike bars, are much more common in Seyferts than in normal disk galaxies. However, Seyferts are typically early-type (SO-Sb) disk galaxies (Heckman 1978, SSS), as are ringed galaxies (e.g. Heckman 1978). Thus, SSS should have compared Seyferts to normal early-type (rather than both early and late-type) disk galaxies. We have done this comparison and find only 15% of the normal galaxies have external rings, while SSS find such rings in 43% of their sample of Seyferts. There is no significant excess of inner rings in Seyferts.
Ring-like structure may frequently be a result of a global nonaxisymmetric gravitational potential (e.g. an oval distortion or a nearby companion galaxy). If we take either a bar or ring to indicate a nonaxisymmetric disk potential, we find that all but two of the 28 Seyferts studied by SSS are nonaxisymmetric. The exceptions are Mrk 348, which is known to have a large, ovally distorted H I ring (Heckman et al. 1981c), and IC 5063, which may have no disk at all (Caldwell & Phillips 1981, but see also Danziger et al. 1981).
Model studies by SSS of the structure and evolution of galaxies with nonaxisymmetric potentials show an intriguing result. Under the proper - and fairly restrictive - conditions, galaxies will develop both ring-like structures and a flow rate of ~ 0.5M yr-1 of gas into the nucleus. The disks of Seyfert galaxies will go through phases in which multiple ring-like features change in their relative prominences. Specifically, SSS propose that Seyfert disks have a three-step distribution in radial surface brightness: "(1) an inner disk or ring, (2) an intermediate envelope, bar, or lens, and (3) a faint outer envelope, ring, or pseudoring." (Kachikian & Weedman 1971 suggested a similar description of type 2 Seyfert galaxies based on less uniform data.) Spiral arms appear transiently in the earlier phases of the system's evolution. Later the disk becomes amorphous while the second and third ring components grow in prominence.
For type 1 Seyfert galaxies, SS have carried these ideas further and present evidence that the nucleus evolves along with the disk; As the disk becomes increasingly amorphous, the width of the broad Balmer emission lines in the nucleus increases. This process ends with the production of a powerful double-radio source. While fascinating, this scenario is supported by a very heterogeneous data base in which the objects having the most amorphous disks (and the broadest Balmer lines) are also the most distant objects. The purported correlations are not very clear if only galaxies of comparable redshifts are examined, and thus seeing-related and Malmquist bias problems are likely. Nonetheless the concepts proposed by SS and SSS are of sufficient interest to pursue observationally and theoretically.