| Description: | The angular rate at which bar and spiral structures rotate--the pattern speed--is a fundamental characterization of the influence these structures have on the evolution of galaxy disks. Since this speed determines the rate at which gas and stars encounter these structures, it is essential for understanding their impact on star formation, the processing of the interstellar medium, and the distribution of metals, as well as on the fueling of central starburst and AGN activity. Yet, issues such as the relation between bars and spirals and the persistence of spiral patterns remain observationally unresolved. I will describe recent efforts to address these issues through the characterization of the angular speeds of patterns and their possible radial variation. The Radial Tremaine-Weinberg (TWR) Method, a generalized version of the Tremaine-Weinberg method for observationally determining a single, constant pattern speed, allows the pattern speed to vary arbitrarily with radius. I will demonstrate the successful measurement of spiral winding and both bar and spiral pattern speeds in a single disk from applications of the TWR method to simulated galaxy data sets. I will then present results for a small sample of nearby galaxies using observations of the ISM (both atomic and molecular phases) as a kinematic tracer. This small sample reveals a surprisingly diverse set of pattern speed behaviors, with the first direct evidence for the presence of resonant coupling of multiple distinct patterns found in some systems, and the measurement of a simple single pattern speed in others. |
