Kinematic methods of spiral nature identification: limitations and applicability

Abstract The nature of spiral arms in galaxies remains debated, with several competing formation scenarios, including density waves, kinematic spirals, dynamic spirals, and recurrent modes, that are difficult to distinguish observationally. We tested two kinematic methods — the phase reversal method and the velocity harmonic dominance method — using toy models and mock data from TNG50 simulations. For bar resonances, the phase reversal method is strongly orientation-dependent and yields reliable corotation estimates only when the bar is aligned with the disc minor axis. For spiral arms, the phase reversal method does not always yield reliable results and should be used in tondem with another kinematic method. The velocity harmonic dominance method is more robust: it distinguishes between density wave, kinematic, and dynamic spiral patterns, although it still shows some sensitivity to orientation. Uncertainties in the rotation curve do not strongly affect its results. We also apply both methods to the observed grand-design spiral galaxy NGC 3938 as an illustrative example; the results suggest the presence of multiple spiral modes, consistent with the recurrent modes scenario and this interpretation relies on different indirect evidence. Based on our findings, we propose a strategy for classifying spiral structure in observed galaxies that combines multiple independent tests. Applying this analysis to an extended sample will allow us to validate its predictive power and to investigate the diversity of spiral formation scenarios.