Resonant structure in the disks of spiral galaxies, using
  phase-reversals in streaming motions from 2D H{\alpha} Fabry-Perot
  spectroscopy
  In this article we introduce a technique for finding resonance radii in a
disk galaxy. We use a two-dimensional velocity field in H{\alpha} emission
obtained with Fabry-Perot interferometry, derive the classical rotation curve,
and subtract it off, leaving a residual velocity map. As the streaming motions
should reverse sign at corotation, we detect these reversals, and plot them in
a histogram against galactocentric radius, excluding points where the amplitude
of the reversal is smaller than the measurement uncertainty. The histograms
show well-defined peaks which we assume to occur at resonance radii,
identifying corotations as the most prominent peaks corresponding to the
relevant morphological features of the galaxy (notably bars and spiral arm
systems). We compare our results with published measurements on the same
galaxies using other methods and different types of data.
