Abstract
A coherent exposition of the density wave theory of galactic spirals is presented in a recent monograph. It is centered on the working hypothesis of quasi-stationary spiral structure, a possibility first proposed by Bertil Lindblad. This hypothesis has since been found to be widely applicable in a number of physical contexts, including the explanation of the Hubble classification system and other categorical classes. Direct empirical support of this hypothesis has been provided especially by the regularity of the infrared images frequently observed in a number of galaxies (e.g., NGC 309), and by the observed amplitude modulation along the spiral arms (e.g., M51, M81, and NGC 1300). The present paper is a brief review of this theory with further clarification of the fundamentals and of certain specific issues raised in the literature. On the theoretical side, the likelihood for the validity of this hypothesis has been supported by modal studies. Emphasis is placed on the widely observed coexistence of a single regular structure in the Pop II objects and the more complex irregular structures in the Pop I objects, a contrast first discovered by Zwicky many years ago in the main disk of M51. It is pointed out that, in both barred and nonbarred spirals, this basic phenomenon may be understood by noting the fact that the microscale of the collisionless system of Pop II stars--i.e., the diameter of the epicycle--is typically on the same order of magnitude as the observed spacing between the spiral arms. The spiral pattern is thus a very compact structure which is unlikely to respond readily to internal and external disturbances of moderate magnitudes. This apparent robustness in structure is suggested by its observed regularity despite the impact from the coexisting Pop I objects with their strong irregular turbulent motions. This physical picture supports Oort's conjecture of a limited role for tidal interaction in most of the spiral structure of galaxies. It also enables us to place in proper perspective a controversy between two schools of thought, with different emphasis placed on intrinsic mechanisms and on tidal interaction. It is primarily a matter of applicability, or frequency of occurrence of the different scenarios proposed. There is, as yet, little observational evidence presented in the literature that would support the need of an interpretation of the global spiral structure in the galactic disk in terms of a fast evolving structure recently generated from a featureless initial state through tidal interaction. Dynamically, the likelihood for realizing such a scenario is also estimated to be quite low. The above discussions are placed in the context of a general point of view much advocated in current scientific literature; namely, the need to analyze alternative mathematical models for diverse physical contexts in the study of "complex" systems and to judge separately the merits of each model on the basis of its empirical confirmation in an appropriate physical context.
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