Science and Reason: How do galaxies grow?

It's certainly a golden age for research in astrophysics and cosmology. This is largely a result of all the new observational technology -- a number of very powerful ground-based telescopes in addition to Hubble and other space-based instruments.

One area in which new results are appearing frequently is the question of how galaxies form and evolve. See here, for example.

One topic that is rapidly becoming clearer is the question of how large galaxies come about. They aren't born fully grown, nor do they grow gradually. Basically, large galaxies come about through cannibalism.

Galaxy Collisions Dominate the Local Universe

New Haven, Conn. — More than half of the largest galaxies in the nearby universe have collided and merged with another galaxy in the past two billion years, according to a Yale astronomer in a study using hundreds of images from two of the deepest sky surveys ever conducted.

The idea of large galaxies being assembled primarily by mergers rather than evolving by themselves in isolation has grown to dominate cosmological thinking.

One apparent problem with this idea is that there are many large galaxies which appear to be quite old. (See here.) However, there are ways to deal with this:

[A] troubling inconsistency within this general theory has been that the most massive galaxies appear to be the oldest, leaving minimal time since the Big Bang for the mergers to have occurred.

“Our study found these common massive galaxies do form by mergers. It is just that the mergers happen quickly, and the features that reveal the mergers are very faint and therefore difficult to detect,” said Pieter van Dokkum, assistant professor of astronomy at Yale University, and sole author of the paper appearing in the December 2005 issue of the Astronomical Journal.