Clarifying the nature of the spiral structure in galaxies: such different cases of NGC 3686 and M 100

At present, the question remains whether spiral arms are long-lived or not. The two main theories about the nature of the spiral structure (density waves and the theory of dynamical spirals) give different answers to this question. There is a number of evidence in favor of both of them, while convincingly confirming which of them is correct from observations is an extremely non-trivial task. In this paper, using the analysis of the collected sample of corotation radii, we found two spiral galaxies NGC~3686 and M~100, which are expected to have a fundamentally different answer to the question considered. In the first galaxy, a quasi-stationary density wave is clearly detected, and in the second case, the spirals are of a dynamic nature. We validate our analysis independently with model, kinematic, morphological and metallicity distribution analysis. For the first time, such a large number of independent observational evidence collected and convincingly shows that both theories can be valid in real objects.