Gas velocity patterns in simulated galaxies: Observational diagnostics
  of spiral structure theories
  There are two theories of stellar spiral arms in isolated disc galaxies that
model stellar spiral arms with different longevities: quasi-stationary density
wave theory, which characterises spirals as rigidly rotating, long-lived
patterns (i.e. steady spirals), and dynamic spiral theory, which characterises
spirals as differentially rotating, transient, recurrent patterns (i.e. dynamic
spirals). In order to discriminate between these two spiral models
observationally, we investigated the differences between the gas velocity
patterns predicted by these two spiral models in hydrodynamic simulations. We
found that the azimuthal phases of the velocity patterns relative to the gas
density peaks (i.e. gaseous arms) differ between the two models, as do the gas
flows; nevertheless, the velocity patterns themselves are similar for both
models. Such similarity suggests that the mere existence of streaming motions
does not conclusively confirm the steady spiral model. However, we found that
the steady spiral model shows that the gaseous arms have radial streaming
motions well inside the co-rotation radius, whereas the dynamic spiral model
predicts that the gaseous arms tend to have tangential streaming motions. These
differences suggest that the gas velocity patterns around spiral arms will
enable distinction between the spiral theories.
