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Astronomers have spotted a huge cloud of fiery gas speeding through a distant cluster of galaxies. They say it is the biggest object of its kind ever seen.

The gas ball contains more matter than a 1,000 billion Suns, and is plunging through the Abell 3266 cluster of galaxies at about 750 kilometres per second. The fireball is about 3 million light years across, roughly 5 billion times the diameter of the Solar System, and reaches temperatures of tens of millions of degrees.

"The size and velocity of this gas ball is truly fantastic," says Alexis Finoguenov, a physicist at the University of Maryland, Baltimore County, and one of the scientists who made the find.

Finoguenov says that the fireball is likely to become a massive building block in the growing cluster, which contains hundreds of galaxies. The discovery is described in the Astrophysical Journal¹.

It rattles their brains

The team surveyed Abell 3266 using the European Space Agency's XMM-Newton probe, an orbiting telescope that observes the skies at X-ray wavelengths. The four-tonne satellite, launched in 1999, uses more than a hundred wafer-thin metal mirrors to pinpoint where the X-rays are coming from. The spectrum of the highly energetic radiation can be used to calculate the temperature of a source and to ascertain some aspects of its composition.

The giant gas ball is mostly made from hydrogen, but contains much larger amounts of heavier elements than the surrounding cluster, suggesting that it is a stranger to the area.

The ball probably originated in a subcluster of galaxies that has merged into Abell 3266, says Norbert Schartel, who leads XMM-Newton's scientific team from the European Space Astronomy Centre at Villafranca del Castillo, Spain. The fast-moving molecules of gas keep the ball's temperature so high that it cannot collapse to form individual stars, he adds.

What a thrill

Why, then, does the superheated fireball not fly apart? According to Mark Henriksen, also a member of the research team at UMBC, the answer is dark matter.

Dark matter is thought to make up more than 80% of the Universe's mass. Being dark, it is invisible and has never been identified: but astronomers believe that its gravitational pull helps to explain why all sorts of large-scale structures, from individual galaxies to superclusters that contain thousands of the things, don't fly apart. The fireball's coherence might be explained in the same way. "Dark matter is the gravitational glue holding the gas ball together," says Henriksen

Schartel says further data from XMM-Newton should allow astronomers to make computer simulations of the galloping gas, which could reveal the distribution of dark matter within the cluster. The cluster's gravitational fields are stripping material away from the burning ball, which loses a Sun's worth of mass ever hour, they estimate. At that rate it will be all used up in 100 million years or so, which is remarkably quick by galactic standards. In true rock'n'roll style, the great ball of fire is living fast and dying young.

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