NATURE OF NUCLEAR RINGS IN UNBARRED GALAXIES: NGC 7742 AND NGC 7217

We have studied an unbarred Sb galaxy with a nuclear star-forming ring, NGC 7742, by means of two-dimensional spectroscopy, long-slit spectroscopy, and imaging and have compared the results with the properties of another galaxy of this type, NGC 7217, which was studied by us earlier. Both galaxies have many peculiar features in common: each has two global exponential stellar disks with different scale lengths, each possesses a circumnuclear inclined gaseous disk with a radius of 300 pc, and each has a global counterrotating subsystem, a gaseous one in NGC 7742 and a stellar one in NGC 7217. We suggest that a past minor merger is the probable cause of all these peculiarities, including the appearance of nuclear star-forming rings without global bars; the rings might be produced as resonance features by tidally induced oval distortions of the global stellar disks.