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 ILR and a small effect
at the OLR. The method works well with observations at high spectral and
angular resolutions, and is suited to the analysis of 2D velocity fields in Ha
from Fabry-Perot spectroscopy, (though it is also applicable to fields in 21cm
emission from HI, or to CO emission lines). The observations are mainly from
the GHASP spectrometer data base, taken on the 1.93m telescope at Haute
Provence, plus some data from the GHaFaS spectrometer on the 4.2m WHT at La
Palma. 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. We have derived the
resonance curves: Omega, Omega+/-kappa/2, Omega+/-kappa/4 against radius for
all the galaxies, and used them to derive the ILR, the OLR, and the two 4:1
resonances for each corotation in each galaxy. This led us to discover a
pattern in over 70% of the sample: given two pattern speeds, say Omega1 and
Omega2, the OLR of Omega1 coincides with the corotation of Omega2, and the
inner 4:1 resonance of Omega2 coincides with the corotation of Omega1. 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 five, and even four times
in one galaxy. We also compute the ratio of corotation radius to bar length
where we have good enough image quality, finding a mean value of 1.3, and a
shallow increase towards later type galaxies.
