Titanic Collision Seen Between Distant Galaxy Clusters

Using CSIRO's Australia Telescope, PhD student Melanie Johnston-Hollitt of the University of Adelaide has found 'wreckage' indicating that two giant clusters of galaxies have collided and merged.

The finding changes astronomers' views of how clusters and individual galaxies evolve.

"Space is big. The chance of things running into each other is small," says Professor Ron Ekers, Director of the Australia Telescope National Facility. "Until now there has been only weak evidence that clusters might collide."

Such collisions may also create ultra-high-energy cosmic rays, the origin of which has been a mystery.

The research will be discussed today [Tuesday 6 Feb] at a workshop on galaxy clusters at the Australia Telescope National Facility headquarters in Sydney.

Clusters are big groups of galaxies, all held huddled together by gravity. Ms Johnston-Hollitt has been studying a cluster of about 500 galaxies called Abell 3667, which lies 700 million light-years away [at a redshift of 0.055].

Abell 3667 appears to be a large cluster that has run into a slightly smaller one. The key evidence is a pair of arcs of radio emission that straddle the cluster, 12 million light-years apart. (On the sky, the distance between them is about twice the diameter of the Moon.)

The collision and its aftermath are like "the Titanic hitting an iceberg," says Ms Johnston-Hollitt. "Afterwards you see only ripples and bits of wreckage, but that's enough to show that there's been a collision."

Radio arcs of the kind seen in this cluster are very rare. The first ones found were thought to be ghostly remains of dead, dissipated galaxies, and were dubbed 'relics', Ms Johnson-Hollitt explains.

Professor Ron Ekers is one of the people who worked on the first known examples of these objects. "It's very satisfying to see that these are markers of important dynamic processes," he says, "not the Cheshire-cat grins of dying galaxies."

Theorists had predicted that galaxy clusters would free-fall together at thousands of kilometres a second, smacking into each other and producing huge shock waves in the thin hot gas that fills the space between the galaxies.

In 1999 researcher Kurt Roettiger, then of University of Missouri-Columbia, and colleagues demonstrated that the shock waves would produce large arcs of radio-emitting particles on the outskirts of the cluster, like those of Abell 3667.

Cluster collisions release the largest amount of energy in a single event since the Big Bang [1057 joules].

They might explain one of the outstanding mysteries in astronomy: the origin of ultra-high-energy cosmic rays [1015 -1021 eV], which amazed astronomers and physicists when first detected on Earth. "No process in our Galaxy can make them," says cosmic ray researcher Dr Roger Clay of the University of Adelaide, one of Ms Johnston-Hollitt's thesis supervisors. "Perhaps shock waves in merging clusters power them up."

The radio arcs in Abell 3667 were first detected with the Molonglo Observatory Synthesis Telescope (MOST) of the University of Sydney. "MOST is an excellent instrument for finding these objects because they emit strongly at the longer wavelengths that MOST works at," says Dr Richard Hunstead of the University of Sydney, another of Ms Johnston-Hollitt's thesis supervisors.

But imaging the arcs in detail required the power of the Australia Telescope. "They are big on the sky but extremely faint," Ms Johnston-Hollitt says. "The Australia Telescope is very sensitive to faint objects and can make wide field images by 'mosaicing' together many smaller views."

"The Australia Telescope picture is the first high-resolution image of such a radio structure. It shows stringy filaments of radio emission that have never been seen before in this kind of source, and which no-one has been able to explain."

Ms Johnson-Hollitt has presented her research at several international meetings, including last year's General Assembly of the International Astronomical Union, astronomy's equivalent of the Olympics.