From the website Starts with a Bang, we have a lovely video (based on computer simulations) depicting the formation of a galaxy similar to ours beginning at the Big Bang and evolving up to the present (“Gyrs” on the lower right represent the passage of billions of years).
(Video credit: Fabio Governato et al./U. of Washington/NASA Advanced Supercomputing.)
Mergers of galaxies are common in their evolution. This movie shows the evolution of a galaxy with similar mass to our own Milky Way, commencing shortly after the Big Bang. The simulation is in a fully cosmological setting, according to our knowledge of Big Bang cosmology. This particular galaxy has a rich merging history, including a major merger at redshift of ~1, i.e. at a time when the Universe was almost half its current age. A large disk reforms from gas left over after the merger, and from subsequent gaseous accretion.
I’d be remiss if I didn’t add that Ethan Siegel, creator of Starts with a Bang, presents this simulation to tout a worthy project: The Charity Engine, in which you can donate some of your own computer’s time to help perform scientific calculations like those involved in the galaxy simulation—and help good charities at the same time.