|Annu. Rev. Astron. Astrophys. 2004. 42:
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3.2. Oval Galaxies
A strong bar has an axial ratio of ~ 1/5 and a mass of ~ 1/3 of the disk mass. In this section, we discuss unbarred but globally oval galaxies in which the whole inner disk has an axial ratio of ~ 0.85. Ovals are less elongated than bars, but more of the disk mass participates in the nonaxisymmetry. As a result, barred and oval galaxies evolve similarly.
Strongly oval galaxies can be recognized independently by photometric criteria (Kormendy & Norman 1979; Kormendy 1982a) and by kinematic criteria (Bosma 1981a, b). The diagnostics are illustrated in Figure 9.
Figure 9. Criteria for recognizing strongly oval but unbarred galaxies shown schematically at left and with observations of NGC 4151 at right. This figure is adapted from Kormendy (1982a). The NGC 4151 H I velocity field is from Bosma, Ekers, & Lequeux (1977a).
Brightness distributions: In prototypical ovals, the disk consists of two nested ovals, each with a shallow surface brightness gradient interior to a sharp outer edge. The inner oval is much brighter than the outer one. The two "shelves" in the brightness distribution have different axial ratios and position angles, so they must be oval if they are coplanar. But the flatness of edge-on galaxies shows that such disks really are oval. Warped disks are common, but they occur at lower surface brightnesses.
Nested ovals in unbarred galaxies are analogous, in barred galaxies, to lenses with embedded bars interior to outer rings. For the purposes of this paper, lenses in early-type galaxies and oval disks in late-type galaxies are functionally equivalent. Both are elliptical shelves in the disk density, and both are nonaxisymmetric enough to drive secular evolution.
Besides NGC 4736 (Fig. 2) and NGC 4151 (Fig. 9), oval disks illustrated in the Hubble Atlas (Sandage 1961) include NGC 4457 (Sa), NGC 3368 (Sa), NGC 4941 (Sa/Sb), NGC 1068 (Sb), NGC 210 (Sb), NGC 4258 (Sb), NGC 5248 (Sc), and NGC 2903 (Sc). Their similarity to barred galaxies can be seen by comparing the two shelves in their brightness distributions with similar ones in NGC 1291 (Fig. 2), NGC 3945 and NGC 3081 (Fig. 5), and, in the Hubble Atlas, NGC 2859 (SB0), NGC 5101 (SB0), NGC 5566 (SBa), NGC 3504 (SBb), and NGC 1097 (SBb).
Kinematics: Velocity fields in oval disks are symmetric and regular, but (1) the kinematic major axis twists with radius, (2) the optical and kinematic major axes are different, and (3) the kinematic major and minor axes are not perpendicular. Twists in the kinematic principal axes are also seen when disks warp. Warps in H I disks are common, but Bosma (1981a, b) points out that they happen at larger radii and lower surface brightnesses than oval structures, which are obvious in Figures 2 and 9 even at small radii. Also, observations (2) and (3) imply ovals, not warps.
As pointed out in Kormendy (1982a), the photometric and kinematic criteria for recognizing ovals are in excellent agreement. These strong ovals are expected to evolve similarly to barred galaxies, because the nonaxisymmetry in the potential is similar to that in barred galaxies. In fact, many simulations of the response of gas to "bars" actually assumed (presumably for computational convenience) that all of the potential is somewhat oval rather than that part of the potential is strongly barred and the rest is not. NGC 4736 is representative of the many unbarred but oval galaxies with strong evidence for secular evolution (see Figure 8 for star formation and Figure 17 for dynamical evidence).
So strongly oval galaxies are readily recognizable. Many are classified SAB; some are SA. However, statistical analyses of large samples of galaxies show that even unbarred disks are slightly oval. The scatter in the Tully-Fisher relation implies that the ellipticity in the potential that controls the disk lies in the range 0 - 0.06. (Franx & de Zeeuw 1992). The corresponding axial ratio of the density distribution is 0.84 - 1.0. Analyses of the velocity fields of individual galaxies give similar results (e.g., Andersen et al. 2001). And, in a study of 18 face-on spiral galaxies using K'-band photometry, Rix & Zaritsky (1995) showed that the typical disk has axial ratio 0.91. Not surprisingly, typical disks are more circular than easily recognized ovals. But they are not round. This is plausible, since disks live inside cold dark matter halos that are predicted to be very triaxial (Frenk et al. 1988; Warren et al. 1992; Cole & Lacey 1996). We now need an investigation of how much secular evolution is driven by the above, small nonaxisymmetries.