7.5. Ringed and lensed galaxies
Revisions to the classification of ringed and lensed galaxies in the de Vaucouleurs system are discussed by Kormendy (1979) and Buta (1986a, 1989), the former based on a survey of 121 barred galaxies and the latter on the CSRG discussed above. These cover mostly the recognition of lenses as distinct from rings and account for variations in the morphology of pseudo-outer rings in barred galaxies.
A small number of especially well-defined ringed nonbarred or weakly-barred S0 and S0/a galaxies, NGC 3081, 7187, 7020, and 7702, from the CSRG has been studied by Buta (1990a, b, c, 1991). These galaxies highlight how high contrast rings are often associated with galaxies which are not classified as SB or even SAB. The rings are discussed in terms of resonances and the secular evolution of bars idea proposed by Kormendy (1979).
Barred galaxies have received much attention the past 15 years, but only recently have the properties of the bar form been analyzed in detail. Elmegreen and Elmegreen (1985) have identified two subcategories of bars which are distinguished by their photometric properties: "flat" bars, where the intensity along the bar length is roughly uniform; and "exponential" bars, where the intensity along the bar declines exponentially. There is a rough correlation between these bar types and Hubble type: flat bars are prevalent among early-type SB galaxies while exponential bars are prevalent among later type SB galaxies. The Elmegreens discuss these differences in terms of orbit resonances in the disk plane.
Another line of morphological work concerns the shape of bars. The bars of a significant fraction of early-type galaxies appear to be somewhat rectangular in shape (e.g., Athanassoula et al., 1990; Ohta, Hamabe, and Wakamatsu, 1990), while the bars of some late-type SBm spirals can be somewhat triangular (Odewahn, 1989). A particular striking example of a rectangular bar with oval "ears" has been found in the southern outer-ringed galaxy NGC 7098 (Crocker and Buta, 1991, in preparation).