2.5. Some Applications of Physical Morphology
2.5.1. Oval Disks
Examination of galaxy photographs shows that many disks are globally oval, even in apparently unbarred galaxies (Kormendy 1979a, 1980; Kormendy and Norman 1979). This property is not explicitly recognized in existing classification systems, although some morphological features characteristic of ovals are described, e.g., in Sandage (1961).
Photometric criteria for identifying oval disks are as follows (see Figures 2, 4 and 27). In well developed examples the brightness distribution consists of a series of nested elliptical regions, each of which has a shallow brightness gradient and a sharp outer edge, and each of which is much fainter than the one immediately interior to it. Generally there are two such regions outside the bulge, sometimes there are more. The outer oval can be a fairly featureless disc, or an outer ring (NGC 4736), or a pair of spiral arms which almost close after half of a revolution to form a pseudoring. The important point is that the axial ratios and position angles of the nested ovals are different, implying that at most one of them can be round if all of them are coplanar. An examination of edge-on galaxies suggests that disks are only rarely warped at the relatively high surface brightnesses (< 25 B mag arcsec-2) of the above features, except in strongly interacting galaxies. Therefore, at least one of the apparently oval regions in the disk is not round. Evidence discussed in Kormendy (1979a) suggests that both features are usually oval. Studies of the apparent axial ratios of disks are consistent with this conclusion (Binney and de Vaucouleurs 1981; Athanassoula et al. 1982). Athanassoula and collaborators find oval disks only in unbarred galaxies. In barred galaxies there is strong evidence that the inner disk (e.g., the lens) is not round (section 5). Typical axial ratios are 0.7 - 0.9. Photometry of oval disks which illustrates the defining features is available for NGC 4736, in Figures 4 and 27 (Simkin 1967; Schommer 1977; Boroson 1981), NGC 4258 (Capaccioli 1974; van der Kruit 1979), and NGC 4941, see Figure 27 (Boroson 1981).
Independent of the above photometric criteria, oval disks can be identified by the properties of their velocity fields. Non-circular motions indicating that some disks are not round were detected even in early kinematic studies. Examples include NGC 1068, 2903, 3504, 4258, 5248 and 6181 (see note 6 to Table 1 of Kormendy and Norman 1979 for references). Bosma (1978, 1981b) has now given explicit kinematic criteria for recognizing ovals, as follows. The velocity field is symmetric about the nucleus, but (1) the kinematic major axis changes position angle with radius, (2) the optical and kinematic major axes are different (e.g., significant rotation is seen about the photometric minor axis), and (3) the kinematic major and minor axes are not perpendicular. Here the kinematic major axis is defined to be the line along which the measured rotation is largest; the kinematic minor axis is the line along which the observed rotation is zero. Examples are given in Figure 4. Generally the above features are also characteristic of warps; we assume that they identify ovals when they occur at surface brightnesses µ < 25 - 26 B mag arcsec-2 at which edge-on disks are usually flat. The kinematic and photometric criteria for identifying ovals are in excellent agreement.
Figure 4. Photographs and H I velocity fields of prototypical, unbarred oval galaxies. Both the photometric and kinematic criteria for recognizing ovals are well illustrated. The upper panels show NGC 4736 at slightly different scales. The velocity field is from Bosma, van der Hulst and Sullivan (1977b). The lower panel shows NGC 4151, from Bosma, Ekers and Lequeux (1977a). Heliocentric velocities are in km s-1. Dashed lines are schematic interpolations. NGC 4736 is further illustrated in the Hubble Atlas, and NGC 4151 in Arp (1977). The brightness profile of NGC 4736 is shown in Figure 27.
Oval disks are important because they constitute (or reflect - see Binney 1978c) non-axisymmetric distortions in the gravitational potential field. These can produce a variety of effects similar to those seen in barred galaxies. Examples include the possibility of driving spiral density waves (section 2.5.3), and the redistribution of disk gas into the nucleus and into rings (sections 2.5.2, 5.4).