SPACE.com -- Galaxies Orbiting Galaxies Create Wakes and Tails

An X-ray image of Abell 160 from the Chandra observatory. Large scale emission from the gaseous medium within the cluster can be seen in addition to smaller regions of emission associated with galaxies and background quasars. CREDIT: David Acreman

An example of an X-ray wake found in Chandra's Abell 160 observation. This cloud of gas is associated with a galaxy which is moving to the left of the image. The motion results in the cloud being distorted towards the right. CREDIT: David Acreman

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	Galaxies Orbiting Galaxies Create Wakes and Tails By SPACE.com Staff posted: 07:03 am ET 08 April 2003

A new observation of a distant cluster of galaxies is helping astronomers understand how galaxies create wakes and tails as they plow through intergalactic gas at supersonic speeds

A new observation of a distant cluster of galaxies is helping astronomers understand how galaxies create wakes and tails as they plow through intergalactic gas at supersonic speeds.

The galaxy cluster Abell 160 was imaged by NASA's Chandra X-ray Observatory, marking the first time the wakes of several galaxies in one cluster have been observed. The results are being presented this week at the UK/Ireland National Astronomy Meeting in Dublin.

Most galaxies congregate in clusters of several to a few thousand. Clusters are pervaded by a large cloud of hot gas, which is held in place by the gravitational attraction of the cluster as a whole. The galaxies move through this gas at high. The resulting ram pressure strips and distorts the gas clouds associated with the galaxies themselves, astronomers say.

When the galaxy is moving supersonically, this is accompanied by a supersonic shock wave like that around a supersonic aircraft.

The new observations show that gas stripped from a galaxy can form a tail that is visible in X-ray observations. The direction in which the tail points indicates the galaxy's path across the sky.

"This information is vital for understanding the way in which galaxy clusters formed, and we can't get it any other way," said doctoral student David Acreman of the University of Birmingham, who led the work.

The observations encompasses 29 of the galaxies in Abell 160, showing that 19 of them appear to be moving on roughly circular orbits while 10 seem to be moving on more radial paths, Acreman said.

"Galaxy clusters are thought to grow as galaxies are pulled onto them by the cluster's gravitational attraction," he said. "This would lead to a preponderance of radial motions."

The preference for circular orbits in Abell 160 suggests the cluster has remained undisturbed for some time, Acreman said, allowing the galaxy orbits to become more circular as the dynamics of the cluster settle down.