The presence of a non-axisymmetric component adds interesting complexity to galaxy dynamics. The fundamental triaxial features in the visible parts of disk galaxies are stellar bars. These are very robust structures: they are produced easily by global instabilities during galaxy formation (Ostriker and Peebles 1973), and then continue to grow stronger as long as the disk is sufficiently cold (Sellwood 1981). As a result, over half of all disk galaxies are barred (de Vaucouleurs 1963). In addition, bars are important because they produce non-axisymmetric forces which are large enough to considerably "stir up" the dynamics. In fact, the belief is growing that bars are the "engines" driving a variety of secular evolution processes in galaxy dynamics (e.g., Kormendy 1981). The important point is that exchanges of energy and angular momentum between individual stars and coherent patterns proceed much more rapidly than two-body relaxation. The latter is too slow in galaxies to be of interest (Chandrasekhar 1960). However, bar-driven secular evolution appears able to make significant changes in galaxy structure over a Hubble time.
The main components found in barred galaxies (de Vaucouleurs 1959a; Sandage 1961; Kormendy 1979a, and Table 1) are illustrated in Figure 40. These include bulges, disks, bars, lenses, and inner and outer rings. Recent work suggests that many of these features are modified or even produced by secular evolution. The properties of the components are reviewed in the following sections. Since we are at a relatively early stage in the study of barred galaxies, many of the questions discussed will be less quantitative than they were in sections 3 and 4. I will nevertheless discuss barred galaxies in some detail, because they provide good examples of morphological and kinematic techniques.
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Figure 40. Examples of barred galaxies. The top panels show an isodensity tracing and photograph of NGC 936, an SB0 galaxy whose kinematics are discussed in section 5.1. The bottom panels (from Kormendy 1981, 1982a) show NGC 2950, an SB(lens)0 galaxy with a triaxial bulge. This bulge is elongated almost horizontally (east-west) in the tracing, while the bar is shown by the outer contour. The high-contrast print from the Palomar Observatory Sky Survey shows that the disk position angle is between those of the bulge and bar. Each pair of photographs and tracings has the same orientation but different scales. |
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Figure 40. continued The top panels show the classic (R)SB(lens)0 galaxy NGC 1291 at low and high contrast. The bottom panels show typical SB(r) and SB(s) galaxies, NGC 2523 (left) and NGC 1300 (right), respectively. Emulsions and exposure times are given in Kormendy (1979a). |