Galaxy Formation Like anything else in the universe, we would like to know how galaxies got that way. There are starting to be observational limits on the redshift of galaxy formation from both directions - stellar evolution and homogeneity of the microwave background. Note again that an observer's and theorist's times of galaxy formation may differ. As Peebles (1989, in The Epoch of Galaxy Formation, Kluwer, p. 1) remarks, an observer would likely date galaxy formation when most of a galaxy's stars are born, while a theorist would time the process from the collapse of a substantial portion of a galaxy's present-day mass out of the Hubble flow. Thus a galaxy might be young in stellar terms but dynamically well-evolved. As just discussedunder galaxy evolution, there are several galactic chronometers that may run at different rates for various kinds of galaxy. The main things we need to account for are the ages of galaxies, their masses, and the distinction between bulges and disks.
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