7. RINGS AND LENSES AS DISTINCT COMPONENTS OF BARRED GALAXY STRUCTURE

Tables 1 and 2 show that if one tries to recognize with classification symbols all of the various types of ring and lens features in a given galaxy, the classification can become disturbingly complex. These types do not yet even account for secondary bars! The large numbers of morphological ``cells'' required to describe some galaxies, and the importance of rings and lenses to barred galaxy structure, led Kormendy (1979a, b) to propose a new morphological approach to galaxy studies. Rather than assigning galaxies to morphological ``cells'' as in Figures 6 and 7, or in Table 2, he proposed identifying a small number of building blocks, or ``distinct components'', in the mass distribution that might be most significant, and to characterize galaxy structure by the way these components may form, evolve, and interact over the lifetime of a system.

The main components are bulges, disks, bars, lenses, and inner and outer rings. Spiral structure was not considered by Kormendy to be a distinct component but rather an aspect of the disk because it represents a reversible perturbation from an equilibrium state (i.e., if the generator of density waves, such as a passing companion, is removed, an equilibrium non-spiral state may be regained). Although rings are intimately connected with spiral structure, as we have amply noted, they result from dissipation processes that slowly accumulate gas into more permanent features. If the perturbation that caused a ring disappears, the ring may survive for a long time afterward. Thus, rings are justifiably distinct components in spite of their close connection to spiral structure. Using the ``distinct component'' approach as a basis of deductions concerning dynamics, Kormendy proposed that lenses result from bar dissolution. This idea, as well as others concerning the origin of ovals, is discussed in more detail in section 15.1.