Messy galactic mergers lead to delicate spiral shapes



Most of the spiral galaxies that decorate our universe have emerged from surprisingly violent pasts, says a new study. They grew their delicate spiral arms after being mashed into a pulp by vast collisions.
This conclusion comes from comparing two snapshots of the universe. A group led by Francois Hammer of the Paris Observatory first took a sample of relatively nearby galaxies observed by the Sloan Digital Sky Survey in New Mexico. It shows that most large galaxies today are disc-shaped, with bright new stars forming along spiral arms – similar to our galaxy and its neighbour Andromeda.
The picture was very different when the universe was about half its present age. To look back to that time, Hammer's team used the GOODS survey, a set of data from Hubble and other space-based observatories. It includes very distant galaxies whose light has been stretched and reddened by the expansion of the universe. The galaxies in Hammer's sample are seen as they were 6 billion years ago.
Among the large galaxies in this era, Hammer's group finds that there are far fewer tidy spirals, and far more with peculiar unclassifiable shapes. Those anomalous galaxies must evolve into spirals to explain the galactic distribution we see today.
Even at first glance, those anomalous galaxies look as though something has smashed them up, and indeed Hammer's simulations show that their shapes and internal motions can be created by two smaller galaxies collidingMovie Camera.

Butterfly from pupa

This is a surprise because spirals need a good supply of cool gas to form new stars, and a big collision should disperse and heat up the gas. Astronomers thought that major collisions would produce relatively dead elliptical galaxies with little new star formation.
The solution in Hammer's simulations is to load the progenitors with gas. If there is more gas than stars, then enough cool gas can be salvaged from the collision to form a disc and then spiral arms, like a butterfly being formed out of a pulpy pupa. "It takes 2 to 3 billion years," says Hammer.
From the high proportion of galaxies that were going through this painful change 6 billion years ago, he calculates that it must have happened to almost all of today's spirals.
So was our own our Milky Way a smashed-up mess 6 billion years ago? It turns out not. We are one of the few exceptions: a spiral that has rotated serenely, undisturbed except by the impact of a few small dwarf galaxies, for 11 billion years – most of the age of the universe.
Journal reference: arxiv.org/abs/0903.3962