The visual classification is important to group the galaxies in families, so that one may study the different formation processes (types of interactions) to understand their dynamics and structural evolution. In other words, each family should be triggered by one or more physical phenomena such as close encounters, collisions, or mergers. There are some exceptions to those scenarios, such as some very symmetric features in ring galaxies that may not be explained by merging. An example is the case of HRG 54103, for which an alternative explanation may be that material rotating with different periods would settle in the ring (Barnes 1996). What about the different kinds of progenitors and interaction scenarios of PRG formation? Numerical models can explain some morphologies either by close encounters (gravitational perturbation) or collision (physical contact of some components of the interacting galaxies) - see e.g. Toomre (1969, 1977, 1978, 1985); Toomre & Toomre (1973); Lynds & Toomre (1976).
In summary, one may divide the ring galaxies into two main classes, namely "NRG" (ringed objects due to resonant trapping of matter) and "PRG" (often collisional).
The projected ring families displayed in Table 1 (click here) are sufficiently broad to include all known PRG. We have purposely included O-type galaxies because they are morphologically interesting, in contrast to the other NRG. Over 20000 galaxies have information on classification in the RC3 (de Vaucouleurs et al. 1991), but a few show ring-shaped structures which have not been cited in the catalogue. In addition, there are galaxies which have not been classified because many have unusual morphologies due to interaction. How many dramatically distorted interacting systems show ring or pseudo-ring structures? There have been no systematical studies in the literature yet. In our sample, a few objects of this class with definite rings have been included in the Irregular family.
The warped rings are objects undergoing strong interaction, they may exhibit perturbed pseudo-rings, some probably being interaction-remnants. The smooth elliptical rings show both centered and off-centered bulges; small evidence of probable star forming regions is present in some objects, in contrast to the richness of HII regions displayed by the knotted elliptical rings. The solitaire objects show either smooth or smooth plus crispy rings, and no relation with the intruder's morphology has been found, which can be elongated, spherical or irregular.
Table 2 displays basic data on the 489 studied galaxies (available in electronic form only, at CDS). The different columns show: (1) the name of the object ("OA" refer to objects found during inspection of the DSS's disks); (2) other designation; (3) and (4) galaxy coordinates for the 2000 equinox (the quoted coordinates are expected to be good to 1 arcsec each; only HRG 00502 has a larger error); (5) the ring family classification; N, s and r? stand for NRG, spiral and probable ring existence, respectively, a blank space meaning the object has not been classified; (6) to (9) for illustrative purposes, the bulge's and ring's major and minor axes in arcsec, respectively; and (10) note running number (see Appendix A, also at CDS). Excluding the N, s and not-classified galaxies, the ellipticals comprise 46% of the quoted PRGs, followed by the irregulars and polars, with 14% and 12%, respectively (see Table 3 (click here)). The axe's measurements have been performed with a micrometer microscope at the Pontificia Universidad Católica de Chile, with a precision of 0.25 mm.
| Family | Number of objects | % |
| P | 51 | 12.0 |
| P? | 26 | 6.1 |
| HL | 8 | 1.9 |
| HL? | 3 | 0.7 |
| E | 197 | 46.4 |
| E? | 6 | 1.41 |
| I | 59 | 13.9 |
| I? | 3 | 0.7 |
| CS | 8 | 1.9 |
| CS? | 1 | 0.2 |
| r? | 63 | 14.8 |
In this first stage, this work has been concerned only with the projected morphology. Structural studies based on detailed surface brightness measurements on direct CCD images of a selected sample of PRG are in progress; these will help in disentangling the different structures of PRG, allowing models including orientation to be tested. A spectral classification scheme of Polar rings and spectroscopic studies of other ring families are under development with other collaborators; these results will be published in a series of forthcoming papers.
Acknowledgements
This work was partially supported by the Conselho Nacional de Desenvolvimento Cientıfico e Tecnológico (CNPq, Brazil). The Departamento de Astronomıa y Astrofısica of the Pontificia Universidad Católica de Chile has kindly made available their laboratories and computational facilities. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, Caltech, under contract with the National Aeronautics and Space Administration. We would like to thank the anonymous referee for his helpful comments and suggestions.