Observational Evidence for Shearing Spiral Patterns in Galaxies

A persisting question in theories of galactic spiral structure is whether the spiral patterns are rigidly rotating density waves,
shearing material arms, or perhaps some combination of both.  In this talk I will discuss the results from my recently finished PhD research that is
helping to answer this question.  The focus of my PhD is to test for spiral pattern rigidity by directly measuring the angular rotation rate of
the spiral pattern, or pattern speed. The method for measuring the pattern speed is based on a new solution of the Tremaine-Weinberg (TW) equations.
These are integrated forms of the continuity equation that relate the pattern speed to the observable kinematic and density properties of a
pattern tracer. The goodness of fit for solutions using a constant pattern speed model are compared with those using pattern speed models that are
functions of radius to infer the radial behavior of the pattern speed. The results for a sample of spiral galaxies show that their patterns are
shearing at a rate that is similar to that of the material. There are also no clear indications for the wave resonances that are traditionally used
for identifying the pattern speed. The results are consistent with transient, recurring, sheared gravitational instabilities. This type of
shearing pattern is regularly observed in spiral galaxy simulations. Future research plans in galactic dynamics and evolution are also
discussed.