A new observation of the galaxy cluster Abell 160 is giving astronomers
important insight into the motion of galaxies as they plough through tenuous
gas in space at supersonic speeds. Observations made with the Chandra X-ray
Observatory reveal the wakes they create behind them. These wakes, or tails,
are a unique source of information about the movements of galaxies in
clusters, which is helping astronomers understand how the structures of
galaxies in the universe have been assembled. This is the first time that
the wakes of many galaxies in one cluster have been observed. The results,
obtained by an international team of astronomers from the UK and the US,
will be presented by doctoral student David Acreman of the University of
Birmingham (UK) at the UK/Ireland National Astronomy Meeting in Dublin on
Tuesday 8 April.

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).