18. Extragalactic astronomy¶
In the previous chapter, we saw that the formation and evolution of the dark-matter halos that host galaxies is conceptually simple, because gravity is the only relevant force. Many of the properties of dark-matter halos can, furthermore, be determined from relatively simple, linear perturbation theory. In detail, the complexity of the evolution of the many-body system of dark-matter particles requires \(N\)-body simulations to fully resolve (see Chapter 12.4) , but even these are quite straightforward: as long as numerical limitations are taken into account, the predictions from such simulations have converged between different codes used to run these simulations. But to understand the formation and evolution of the stellar and gas components inside a dark-matter halo—what we refer to as “galaxies”—and to connect the dark-matter halos from the previous chapter to observations, we need to consider the far less well understood, but long list of topics such gas cooling, star formation, stellar and chemical evolution, galactic dynamical evolution, cosmic-ray transport, etc. In keeping with the philosophy of this book, we will not attempt to give a full overview of each of these fields, but will go over some of the main techniques and results from these subjects as they relate to galaxy formation and evolution. We start in this chapter with a focus mainly (but not exclusively) on observations, discussing how we go from direct observational properties to fundamental galactic properties such as stellar mass, gas mass, star-formation rate and history, morphology, and chemical content, and giving an overview of the empirical picture of galaxy formation and evolution that emerges from these. In the next chapter, we will start diving into a more theoretical picture of galaxy formation.
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