THE OUTER LINDBLAD RESONANCE AND THE MORPHOLOGY OF EARLY TYPE DISK GALAXIES

The morphology of outer rings and pseudorings in SB and SAB galaxies includes a number of distinct categories that bear a remarkable resemblance to the gaseous rings which have developed near the outer Lindblad resonance in n-body models of barred spirals. This paper displays these categories using CCD images and color index maps of 22 galaxies from the Catalogue of Southern Ringed Galaxies (CSRG). The prominence of the OLR morphologies in these and many other galaxies provides an important handle on pattern speeds of bars and ovals, as well as insights into internal dynamics. It also provides an important distinction to be considered when comparing early and late-type galaxies. An interesting possible correlation between the presence of nuclear star formation and outer ring/pseudoring morphology emerges from the new images. In a class of rings where the arms wind approximately 180-degrees with respect to the ends of a bar or oval, 9 out of 11 of our examples display blue nuclei or circumnuclear rings in a B - I color index map. This contrasts with a class where the arms wind approximately 270-degrees with respect to the ends of a bar or oval, for which only 3 out of 11 of our examples display nuclear star formation. We also find a similar correlation regarding the presence or absence of a clear dust lane pattern within the region of a bar or inner oval. The reasons for these correlations are not obvious, and a larger sample is needed to verify them. An interesting finding is that the triple-ringed S0/a galaxy NGC 1326 contains a remarkable pattern of dust lanes in the region of its weak bar that bears no resemblance to any model. We also present images of eight additional CSRG galaxies which display morphological details of special interest. For example, ESO 566 - 24 is an exceptionally regular four-armed ringed barred spiral that is rare in the CSRG, while ESO 565 - 11 is an unusual object with a large nuclear ring of blue knots, a peculiar bar, and atypical inner and outer pseudorings. Other cases are described in an Appendix.