METRIC CHARACTERISTICS OF NUCLEAR RINGS AND RELATED FEATURES IN SPIRAL GALAXIES

The metric properties of nuclear rings, pseudorings, spirals, and bars are discussed in the context of resonance theory and are used to gauge the range of sizes for such features. We find that the nuclear rings and pseudorings of strongly barred galaxies (types SB and SAB) have a wide range in linear diameter, from about 0.4 kpc in ESO 566 - 24 to 3.8 kpc in ESO 565 - 11. The mean diameter is about 1.1 kpc for a sample of 20 objects. We suggest that there are clear analogues of the nuclear rings of strongly barred spirals in both weakly barred (SAB and SA.B) and nonbarred (SA) spirals. The size ratios of nuclear rings with respect to outer rings and pseudorings, as well as morphological characteristics, are best explained if outer rings and pseudorings are linked to the outer Lindblad resonance, and nuclear rings, pseudorings, and spirals are linked to the inner Lindblad resonance. These resonance identifications are consistent with most recent deductions. Nuclear bars are often associated with nuclear rings and spirals, but also can exist independently of such features. We present a list of 13 double-barred galaxies and discuss the significance of the phase angle of the nuclear bar with respect to the primary bar.