Posted on: Friday, September 24, 2004

Now showing: The galactic storm

By Kevin Dayton
Advertiser Big Island Bureau

HILO, Hawai'i — Scientists released images yesterday of a collision of two galaxy clusters as they crash into each other like immense storm systems, a process that releases more energy than anything but the Big Bang itself.

J. Patrick Henry, team leader of the researchers and professor of astronomy at the University of Hawai'i-Manoa, said the galaxy clusters first met about 300 million years ago, and will continue to merge for about another 7 billion years.

To put the event on a somewhat more human scale, scientists have been watching a collection of about 300 galaxies like our own Milky Way galaxy collide with another system of about 1,000 galaxies.

Compared with the overall size of the universe, the collision is taking place relatively close by — about 800 million light years away — and is smashing together systems made up of trillions of stars, scientists said. A light year is the distance light travels in a year, or about 6 trillion miles.

Scientists have known for many years that the collision is taking place in the cluster known as Abell 754, which is in the constellation Hydra.

The scientific team, which included Alexis Finoguenov and Ulrich Briel of the Max-Planck Institute for Extraterrestrial Physics in Germany, made "an exceptionally detailed, panoramic observation" as the gravitational pull of the larger galaxy cluster sucked a smaller cluster into it, Henry said.

That created what astronomers are calling hurricane-like conditions, jolting entire galaxies far from their original paths and generating shock waves of 100-million-degree gas.

One cluster apparently smashed into the other and passed through it, and now gravity will pull the remnants of the first cluster back toward the core of the second, said Finoguenov. During the next few billion years, scientists expect the remnants of the clusters will settle, completing the merger.

Scientists used X-ray observations from the European Space Agency's XMM-Newton satellite observatory to obtain images of the collision and generate temperature and pressure maps similar to the familiar weather maps charting storm systems on Earth.

Galaxy clusters are the most massive objects in the universe, and "what we think is happening is galaxies are falling into this cluster, sometimes hundreds of galaxies fall into the cluster, and it grows bigger and bigger," Henry said.

The images are exciting in part because the actual observations of the collision closely resemble computer simulations scientists developed of such collisions. That suggests astronomers now have a reasonably accurate understanding of the forces at work.

The Milky Way is subject to the same forces. Scientists said our own galaxy is part of a small group moving toward the Virgo Cluster, and will go through a collision of its own in "a few billion years."

Reach Kevin Dayton at kdayton@honoluluadvertiser.com or (808) 935-3916.