Radial distributions of arm-gas offsets as an observational test of
  spiral theories
  Theories of stellar spiral arms in disk galaxies can be grouped into two
classes based on the longevity of a spiral arm. Although the quasi-stationary
density wave theory supposes that spirals are rigidly-rotating, long-lived
patterns, the dynamic spiral theory predicts that spirals are
differentially-rotating, transient, recurrent patterns. In order to distinguish
between the two spiral models from observations, we performed hydrodynamic
simulations with steady and dynamic spiral models. Hydrodynamics simulations in
steady spiral models demonstrated that the dust lane locations relative to the
stellar spiral arms (hereafter, arm-gas offsets) depend on radius, regardless
of the strength and pitch angle of the spiral and the model of the
inter-stellar medium (ISM). In contrast, we found that the dynamic spiral
models show no systematic radial dependence of the arm-gas offsets. The arm-gas
offset radial profile method, together with the other test methods, will help
us to distinguish between the two spiral models in observed spiral galaxies.
