Global Spiral Patterns in Galaxies : A Review of the Density Wave Theory from the Theoretical Perspective

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Global Spiral Patterns in Galaxies: A Review of the Density Wave Theory from the Theoretical Perspective

The density wave theory of galactic spirals is a semi empirical theory consisting of the analysis of physically plausible mathematical models, the synthesis of extensive observational data, and an examination of the applicability of the theoretical predictions of particular models for the interpretation of observational data. In this paper, the theory is reviewed from the general perception of order and chaos in dynamical systems. Prominent among the observational data is the well established morphological classification of galaxies according to Edwin Hubble, the luminosity class of Sidney van den Bergh, and the coexistence of regular and irregular global structures in many galaxies, as was first discovered in the whirlpool galaxy M51 by Fritz Zwicky. The regular structure is prominently observed in red and in infrared, and the irregular in blue and at 21 cm radio frequency. These and other data led to the concept of the existence of an essentially standing wave pattern in the older evolved stars, which in turn acts as a backbone for the global structure of the galaxy as a whole. Dynamically, the existence of irregular structures in the interstellar medium reflects the natural state of turbulent motion in gaseous motion at high speeds. In contrast, the essentially collisionless stellar system has a substantial "microscale" of epicyclic motion that prevents, or smoothes out, irregularities on scales. It is pointed out that the spiral structure in galaxies is but one class of interesting phenomena that crucially relates to universal gravitation. Astronomical phenomena on all scales, from planets and satellites to protostars, stars and galaxies and the whole universe present other challenges to both theorists and observers, representing a wide field of opportunities for fruitful research. At the present time, they are being successfully met in individual cases through the development of new observational techniques to cover all frequencies of electromagnetic radiation and through the development of new analytical and computational methods.