Model-independent Measurement of Spiral Pattern Speeds and their Radial Variation

Astronomy and Astrophysics – Astronomy

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Scientific paper

At the intersection of galactic dynamics, evolution and global structure, issues such as the relation between bars and spirals and the persistence of spiral patterns can be addressed through the characterization of the angular speeds of the 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 review the utility of, and caveats in applying, the method together with a standard regularization technique, compiled through tests of the calculation on N-body simulations. I will also present results for the bright two-armed spiral structure in the grand design galaxy M51, derived using CO observations of the molecule-dominated ISM. Assuming a disk PA=170°, the application has revealed a heretofore un-measured inner spiral pattern speed, with a value significantly higher than conventional estimates. In addition, the radial dependence implied in the TWR solution at this PA, which identifies at least two distinct pattern speeds, suggests a possible resonant link between the inner and outer regions of the bright spiral arms. Future studies to investigate bar and spiral relations, spiral winding, and spiral-spiral mode coupling, should realize considerable success using the TWR method, which has proven to be an invaluable tool for addressing such issues.
This material is based on work partially supported by the National Science Foundation under Grant No. AST-0807032.

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