|Annu. Rev. Astron. Astrophys. 1985. 23:
Copyright © 1985 by Annual Reviews. All rights reserved
Regular disk galaxies sometimes show an outer ring, which (contrary to the case of ring galaxies) is centered on the nuclear bulge. Inside it the disk is well defined and sometimes contains a lens and/or bar. Statistics of the frequency of outer rings as a function of Hubble type show a preponderance of such rings around S0/a galaxies. and early-type spirals (10-15%) and their virtual absence in late-type spirals (32). Good examples of disk galaxies with outer rings are NGC 1291, NGC 2217, and NGC 2859. Two extreme types can be distinguished; broader stellar and truly closed outer rings with a very smooth light distribution and red colors; and "gaseous" (near-) rings consisting of two,or more narrow spiral arms very tightly wound, which on low-resolution plates can look continuous. On higher resolution plates, regions of star formation can often be easily recognized even in stellar rings (e.g. NGC 1543 and NGC 4736).
From the statistics of axial ratios of outer rings (8, 20, 64) it follows that the true shapes of the rings are nearly round, with axial ratios between 0.8 and 1.0. In barred spirals there is a preferred ratio of sizes: The ratio of outer-ring size to bar size is 2.2 ± 0.4 (64), and that of outer-ring size to inner-ring size, if both exist, is similar (8). For ordinary spirals there is no such preferred value (8), though recently Buta (20) identified boundaries of an intermediate zone in a number of SAs, and these "zone edges" or "inner rings" are also a factor of 2.2 smaller in size than the outer rings in those galaxies. We believe these zone edges could be boundaries of mild oval distortions, as is sometimes suggested by H I velocity fields (cf. 13, 14).
Using a simple model rotation curve, it can be shown (8) that the preferred size ratio of rings in barred spirals suggests that the outer ring occurs near the outer Lindblad resonance (OLR) associated with a rotating bar pattern. The bar ends roughly at corotation, and the inner ring occurs near the ultraharmonic resonance just inside corotation. This is in agreement with theoretical expectations (8, 102, 103). Furthermore, outer rings are perpendicular to the bar or oval, as can be argued for some cases with known spatial orientation (64) or statistically from the distribution of apparent position angles between the bar and the outer ring (20, 104).
The response of a gaseous disk to a rotating bar' has been modeled by Schwarz (102, 103, 105) using a cloud-particle scheme with inelastic collisions. As a result of the torque exerted, by the bar, the particles in the region between corotation and the OLR are slowly pushed outward, forming first a spiral, which later evolves via a pseudoring into a true ring at the OLR. If the bar is relatively weak and the gas is initially confined within the radius of the OLR, the ring will be perpendicular to the bar. In this case the ring is said to be inside the OLR, since the angular momentum of the particles within it is less than that of the circular orbit at the OLR. On the other hand, if the gas extends initially beyond the OLR, or if the bar is stronger, the ring will be parallel to the bar. The angular momentum of the ring particles is then larger than that of the circular orbit at the OLR. It was argued (102) that perpendicular rings are not as robust as parallel ones, since if there is even a little gas outside the OLR, it will collide with the particles inside the OLR and disrupt the ring. Yet, as we saw above, the observations seem to show that the outer rings lie preferentially perpendicular to the bar.
In the case of a strong bar, the particles are pushed past the OLR on orbits that are nearly circular in shape. It may thus be expected that outer rings in very strongly barred galaxies are rounder than those in less strongly barred or oval galaxies. Finally, the rate of formation of the ring is faster when the bar is stronger.
Similar results are found if the bar forcing is replaced by a spiral one. In particular, the time scales involved are of the same order, and the outer ring is again formed around the OLR (102, 103). This observation and arguments of continuity along the "families" SB-SAB-SA imply that the outer rings of SA galaxies are also at the OLR. Thus, outer rings will be the end product of the dynamical evolution of all spirals, barred or not. The small percentage of galaxies observed to have outer rings shows that this cannot be true. However, there are several ways out. The mass distribution or the cloud collision law in the models could differ from that in real galaxies, or alternatively the gas between corotation and the OLR could be replenished by mass loss from stars and by infall (102, 103). A numerical simulation, including a replenishment rate of about 2M yr-1, shows that the spiral structure may indeed persist for at least 50 bar rotations. Since the rate of ring formation depends heavily on the bar strength, one can conclude that a strong bar and/or a small replenishment rate will favor ring formation.
Different forms of potentials and different collision laws have been considered (25, 78), with similar results. Since a bar can also accomplish the transfer of material toward the nucleus, as well as create an outer ring, Schwarz's results have been used (124) to relate the morphological characteristics of Seyfert galaxies, which frequently have outer rings, to the spectral properties of the nucleus itself.
If the main lines about the formation of gaseous rings now appear to be well understood (102, 103), this is not the case for diffuse stellar rings. One possible explanation is that these form from the evolution of the narrow gaseous rings. The piling up of gas favors star formation and might result in an initially narrow stellar ring, which subsequently broadens as a result of orbital diffusion (cf. discussion after 6). An alternative scenario, which does not rely on the initial presence of gas, is that stellar rings are the later stages of evolution of stellar spirals due to episodes of bar growth. Yet in numerical simulations, where this mechanism is often observed, purely stellar rings are transients, like their spiral counterparts.