ABSTRACT
Spiral structures are visually the most impressive features in the morphology of galaxies. Most of these spiral structures can reasonably be well understood as the manifestation of global modes of oscillation of galactic disks. The identification and discrimination of these modes is important for the study of dynamical properties of observed galaxies. The method of two-dimensional Fourier analysis presented in this section gives a quantitative basis to achieve this objective. Preliminary application of the present method to M 51 is reported by lye et al, and a full description of the technique and results of application to NGC 4254 are given by Iye et al. They found that the asymmetric spiral structure of NGC 4254 is delineated as a superposition of five-armed and three-armed spiral components on the main one-armed spiral component. Some of the morphological features which have been defined so far by subjective pattern recognition could be quantitatively defined by the proper use of the power spectrum.
