Galactic Rings Revisited. II. Dark Gaps and the Locations of Resonances
  in Early-to-Intermediate Type Disk Galaxies
  Dark gaps are commonly seen in early-to-intermediate type barred galaxies
having inner and outer rings or related features. In this paper, the
morphologies of 54 barred and oval ringed galaxies have been examined with the
goal of determining what the dark gaps are telling us about the structure and
evolution of barred galaxies. The analysis is based mainly on galaxies selected
from the Galaxy Zoo 2 database and the Catalogue of Southern Ringed Galaxies.
The dark gaps between inner and outer rings are of interest because of their
likely association with the L4 and L5 Lagrangian points that would be present
in the gravitational potential of a bar or oval. Since the points are
theoretically expected to lie very close to the corotation resonance (CR) of
the bar pattern, the gaps provide the possibility of locating corotation in
some galaxies simply by measuring the radius rgp of the gap region and setting
rCR=rgp. With the additional assumption of generally flat rotation curves, the
locations of other resonances can be predicted and compared with observed
morphological features. It is shown that this "gap method" provides remarkably
consistent interpretations of the morphology of early-to-intermediate type
barred galaxies. The paper also brings attention to cases where the dark gaps
lie inside an inner ring, rather than between inner and outer rings. These may
have a different origin compared to the inner/outer ring gaps.
