The Milky Way is one of the most typical spiral galaxies, resembling the Andromeda galaxy (M31), and is composed of a nucleus nesting a supermassive black hole, a spheroidal central bulge composed of old stars, flat stellar disk filled with interstellar gas superposed by spiral arms, and a huge dark halo extending half-way to the neighboring galaxy.
The galaxy is the nearest, hence most extensively observed galaxy in detail, however, the special position of the observers located inside the object always poses difficulty in knowing the three-dimensional structure. This difficulty, in turn, forced astronomers to develop sophisticated techniques of observation and analysis. The difficulty is overcome by observing a galaxy from outside, which is in fact extensively done for external galaxies.
In this chapter we describe the method to derive the dynamical structure of galaxies based on the Newtonian theory of gravitation. We learn various methods to measure the mass distributions, and the methods are applied to observed rotation curves of galaxies.
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