As mergers go, this one dwarfs even AOL-Time Warner.

Two of the biggest objects in the known universe, giant galaxy clusters enveloped in hot gas, are seen colliding in the latest images produced by the Chandra X-ray Observatory.

While the collision is tremendous -- each gas cloud contains hundreds of galaxies -- it is more civil than theories had predicted: As with a merger of media giants, shock waves were expected, but none are found in the images.

The distant merger has created a lot of pressure and heat for the participants, however. The surrounding atmosphere of gas is 180 million degrees Fahrenheit (100 million degrees Celsius). Near the center, where the real action is, a relatively "cool" area is created as denser gas radiates its heat away more quickly.

Understanding the process of how galaxy clusters and superclusters form will help scientists figure out how much matter is in the universe and what form it takes, said Wallace Tucker, chief of X-ray Astronomy at NASA's Marshall Space Flight Center.

Tucker, a Chandra project scientist, said the images show the first details of how galaxy clusters are assembled. Though researchers knew that clusters were built over billions of years, through mergers, theories had predicted a more violent collision than what the images indicate.

"The details of this observation show that the theories need a little more work," Tucker told SPACE.com.

Tucker said the lack of a shock wave, and the fact that the images reveal less heat than expected, might indicate the merger is near completion and things have settled down. Or, he said, perhaps the dynamics of cosmic mergers are different than anticipated.

Either way, the observations represent an important step towards learning more.

"Now we can begin to understand the physics of these mergers, which are among the most energetic events in the universe," said Maxim Markevitch of the Harvard-Smithsonian Center for Astrophysics and leader of the international team that analyzed the observations. "The pressure and density maps of the cluster show a sharp boundary that can only exist in the moving environment of a merger."

The two merging clusters together make up a larger galaxy cluster known as Abell 2142. The entire supercluster is 6 million light-years across.

Galaxy clusters grow to such vast sizes by using their gravity to pull in other small galaxies over billions of years. The enormous systems are filled with clouds of hot gas whose mass matches that of all the stars in all the galaxies of the cluster.

Our own Milky Way is part of a group of a dozen or more galaxies, all somewhat isolated but surrounded by the Virgo supercluster. Whether the group might one day join the larger cluster is unclear.

"We're heading towards the center [of the supercluster]," Tucker said. "Maybe in many billions of years we may become part of it."

The new images were made in August, 1999, and released this week. The results will be reported in an upcoming issue of the Astrophysical Journal.