Galactic titans. A search for giant groups of galaxies unveiled 101 new clusters in the distant cosmos, including these two. The cluster on the left (yellow objects) is fully formed, while the one on the right--a billion light-years farther from Earth--appears clumpy and still coalescing.

Chasing Giant Galaxy Clusters

Honolulu--The largest objects held together by gravity in the universe couldn't hide forever. Astronomers have probed deep into the cosmos to find more than 100 massive clusters of galaxies at distances where they previously had seen just a handful. The results, announced here on 8 November at a divisional meeting of the American Astronomical Society, strengthen the scenario that the expansion of the universe will never stop, but instead will accelerate.

Galaxy clusters are assemblages of hundreds or thousands of star systems like our Milky Way. Like cosmic vacuum cleaners, they suck up other galaxies via their relentless gravity. They are common today, but clusters in the depths of space--closer in time to the big bang--had seemed to be rare. However, cosmologists predicted that many giant clusters should have started to coalesce when the universe was just half its current age, or even earlier. Astronomers just needed a way to catch these distant swarms, they said.

A team led by astronomer Harald Ebeling of the University of Hawaii, Honolulu, met the challenge by studying x-rays. Distant clusters aren't obvious in optical light because they are dim and hide behind other stars and galaxies. But look for x-rays, and the clusters pop out like fireflies in a dark field. That's because these "hot balls of gravitationally bound gas," as Ebeling puts it, reach temperatures of 100 million degrees and spew x-rays that can be detected with satellites.

Ebeling's team sifted through public data from an all-sky x-ray survey by the German ROSAT satellite, which ended its 9-year mission in 1999. Starting with several thousand hot x-ray points, the team used other catalogs of objects to narrow the list to unknown and potentially distant sources. Then, they took photos of about 500 promising patches of sky with optical telescopes to search for faint groups of many galaxies. So far, 101 patches have turned out to contain gigantic clusters, most dating to a time when the universe was one-half to two-thirds of its current age. Ebeling expects his team will eventually find about 130 clusters at those distances.

That number jibes well with the latest cosmological models for how the universe has evolved, says astrophysicist August Evrard of the University of Michigan, Ann Arbor. The amount of matter in the universe appears insufficient to stop its growth, and a mysterious "dark energy" may have started to make space expand more quickly in recent time, Evrard says. In that scenario, "You have to form structures at an accelerated rate earlier on in order to produce the number of clusters we see today."

Related sites

More information and images from the University of Hawaii

Background on massive galaxy clusters

The ROSAT x-ray satellite

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