INTERLOCKING RESONANCE PATTERNS IN GALAXY DISKS

We have developed a method for finding dynamical resonances in disk galaxies using the change in sense of the radial component of the in-plane velocity at a resonance radius. Using simulations we show that we would expect to find these changes at corotation radii with a weaker effect at the Lindblad resonances. The method works well with observations at high spectral and angular resolutions, and is suited to the analysis of two-dimensional velocity fields in Ha from Fabry-Perot spectroscopy. We find clear indications of resonance effects in the disk velocity fields of virtually all of the 104 galaxies. The number of resonance radii detected ranges from one to seven, with a median of four. The frequency curves Omega, Omega +/- kappa/2, Omega +/- kappa/4 against radius for all the galaxies led us to discover a pattern in over 70% of the sample: given two pattern speeds, say Omega(1) and Omega(2), the OLR of Omega(1) coincides with the corotation of Omega(2), and the inner 4: 1 resonance of Omega(2) coincides with the corotation of Omega(1). Although the second coincidence has been predicted, a prediction of this double coincidence is not found in the literature. This pattern is found once in 42 of the galaxies, twice in a further 26, three times in 5, and even four times in 1 galaxy. We also compute the ratio of corotation radius to bar length where we have sufficient image quality, finding a mean value of 1.3, and a shallow increase toward later type galaxies.