Most galaxies congregate together in clusters containing anything from several galaxies 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 speeds (up to a few thousand kilometres per second). The resulting ram pressure strips and distorts the gas clouds associated with the galaxies themselves. When the galaxy is moving supersonically, this is accompanied by a supersonic shock wave like that around a supersonic aircraft.

The superb imaging capabilities of the Chandra X-ray observatory enabled unprecedented observations of these shocks and stripping processes which are visible due to the X-rays they emit. 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 David Acreman.

"Abell 160 was an ideal cluster for this kind of study," he says, "because it has not been disrupted by a recent merger with another cluster. It also has the advantage that several bright galaxies are located near its centre where Chandra can view them with a single observation. Our observation encompasses 29 of the galaxies in Abell 160 and we find that 19 of them appear to be moving on roughly circular orbits while 10 seem to be moving on more radial paths."

"Galaxy clusters are thought to grow as galaxies are pulled onto them by the cluster's gravitational attraction; this would lead to a preponderance of radial motions," David Acreman explained. "The preference for circular orbits in Abell 160 suggests we are observing a cluster that has remained undisturbed for some time allowing the galaxy orbits to become more circular as the dynamics of the cluster settle down."

IMAGES

Chandra X-ray images of Abell 160 and individual galaxies in the cluster are available from:

http://www.sr.bham.ac.uk/~da/nam2003/

CONTACT

David Acreman
University of Birmingham
Phone: 0121 414 3652
FAX: 0121 414 3722
E-mail: da@star.sr.bham.ac.uk

OTHER TEAM MEMBERS

Trevor Ponman, Ian Stevens and Irini Sakelliou (University of Birmingham, UK), Michael Merrifield (University of Nottingham, UK), Jason Pinkney (Ohio Northern University, USA) and Michael Ledlow (University of New Mexico, USA).