Galaxies all grew more violent, when they became "teenagers"

This huge panorama of the constellation Cetus shows countless galaxies, all at different distances from Earth, and at different stages of their evolution. It reveals the huge cosmic shift in how galaxies go about growing bigger and bigger.

This image was created using the European Southern Observatory's Very Large Telescope. For the purposes of this current study, astronomers focused on galaxies that were about three to five billion years older than the Big Bang - roughly when the universe was a third its current age, making them the cosmic equivalent of teenagers. And, like a human's teenage years, a lot of galaxies went through massive growth spurts during this period, growing into the sorts of giant galaxies we can see closer to home.

The ESO astronomers have discovered that galaxies seem to have collectively undergone a huge change in how they accumulated mass. For the first few billion years after the Big Bang, galaxies relied on the smooth, continuous flow of gas to grow as peacefully as possible. But then - again, much like actually teenagers - galaxies suddenly impossible to deal with, thrashing around violently and gleefully merging with any other galaxy that was ready and willing. Ever since, galaxies seem to have gotten larger almost entirely through collisions and mergers, rather than gas flow.

According to team leader Thierry Contini, both the gas flow and the merger methods can create lots of new stars, but for reasons that remain mysterious early galaxies preferred the former over the latter, and then around five billion years ago that switched. That isn't the only way in which young galaxies were the opposite of their later counterparts — the team discovered that the earliest galaxies had most of their heavy elements concentrated in their outer regions, which is the opposite of what we see in modern galaxies. Exactly why these things are the way they are remains enigmatic — we've only just been able to see far enough to even realize these things are strange, so explaining them will take a little more time.

Via ESO.