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On the galaxy spiral arms' nature as revealed by rotation frequencies

High resolution N-body simulations using different codes and initial condition techniques reveal two different behaviours for the rotation frequency of transient spiral arms like structures. Whereas unbarred disks present spiral arms nearly corotatingwith disk particles, strong barred models (bulged or bulge-less) quickly develop a bar-spiral structure dominant in density, with a pattern speed almost constant in radius. As the bar strength decreases the arm departs from bar rigid rotation and behaves similar to the unbarred case. In strong barred models we detect in the frequency space other subdominant and slower modes at large radii, in agreement with previous studies, however we also detect them in the configuration space. We propose that the distinctive behaviour of the dominant spiral modes can be exploited in order to constraint the nature of Galactic spiral arms by the astrometric survey GAIA and by 2-D spectroscopic surveys like CALIFA and MANGA in external galaxies.