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Current theories of spiral and bar structure predict a variety of patternspeed behaviors, calling for detailed, direct measurement of the radialvariation of pattern speeds. Our recently developed Radial Tremaine-Weinberg(TWR) method allows this goal to be achieved for the first time. Here wepresent TWR spiral pattern speed estimates for M101, IC 342, NGC 3938 and NGC3344 in order to investigate whether spiral structure is steady or winding,whether spirals are described by multiple pattern speeds, and the relationbetween bar and spiral speeds. Where possible, we interpret our pattern speedsestimates according to the resonance radii associated with each (establishedwith the disk angular rotation), and compare these to previous determinations.By analyzing the high-quality HI and CO data cubes available for thesegalaxies, we show that it is possible to determine directly multiple patternspeeds within these systems, and hence identify the characteristic signaturesof the processes that drive the spiral structure. Even this small sample ofgalaxies reveals a surprisingly complex taxonomy, with the first directevidence for the presence of resonant coupling of multiple patterns found insome 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 moreof the apparently complex physics that lies behind spiral structure.

Uncovering the Origins of Spiral Structure by Measuring Radial Variation in Pattern Speeds