8. The kinematics and dynamics of galactic rotation¶
Now that we have a solid basis in the properties of both spherical and disk mass distributions, we have all of the elements in place for a first in-depth discussion of the kinematics of galactic rotation. We will discuss galactic rotation both in external galaxies and in the Milky Way. The fundamental approximation that we will make in this chapter is that we can measure the velocity of circular orbits in galaxies and use the framework from the previous chapters to relate the circular velocity to the mass distribution. For this reason, we focus on observations of the kinematics of the interstellar medium, which we may assume to be on closed, non-crossing orbits. However, it is important to keep in mind that galaxies have non-axisymmetric components, such as a bar or spiral structure, that cause even gas to deviate from circular orbits. Outside of the inner, bar region, these deviations are small (at the level of 10% or so of the velocity of the gas) and can be averaged over if we can observe galactic rotation over the entire galaxy. These deviations are small enough that they do not impact the major conclusion that we will draw in this chapter: to explain the rotation curves of external galaxies and the Milky Way out to tens of kpc from the center, we require a form of dark matter that cannot be detected through optical, radio, or any other electromagnetic emission.
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