Uncovering the Origins of Spiral Structure by Measuring Radial Variation
  in Pattern Speeds
  Current theories of spiral and bar structure predict a variety of pattern
speed behaviors, calling for detailed, direct measurement of the radial
variation of pattern speeds. Our recently developed Radial Tremaine-Weinberg
(TWR) method allows this goal to be achieved for the first time. Here we
present TWR spiral pattern speed estimates for M101, IC 342, NGC 3938 and NGC
3344 in order to investigate whether spiral structure is steady or winding,
whether spirals are described by multiple pattern speeds, and the relation
between bar and spiral speeds. Where possible, we interpret our pattern speeds
estimates according to the resonance radii associated with each (established
with the disk angular rotation), and compare these to previous determinations.
By analyzing the high-quality HI and CO data cubes available for these
galaxies, we show that it is possible to determine directly multiple pattern
speeds within these systems, and hence identify the characteristic signatures
of the processes that drive the spiral structure. Even this small sample of
galaxies reveals a surprisingly complex taxonomy, with the first direct
evidence for the presence of resonant coupling of multiple patterns found in
some systems, and the measurement of a simple single pattern speed in others.
Overall, this study demonstrates that we are now in a position to uncover more
of the apparently complex physics that lies behind spiral structure.
