Magazine article Science News

Tracing the Architecture of Dark Matter

Magazine article Science News

Tracing the Architecture of Dark Matter

Article excerpt

Stars and galaxies set the night sky aglow, but these glittering jewels account for only a tiny portion of matter in the cosmos. For more than half a century, astronomers have gathered evidence that at least 90 percent of the mass in the universe doesn't emit light. This invisible material, known as dark matter, exerts a gravitational tug, just as stars and galaxies do, but has otherwise remained a mystery.

Now, a team of Japanese astronomers argues that dark matter has another property in common with visible material. The unseen matter forms small lumps that coalesce into bigger lumps, in the same way that visible stars group into galaxies and galaxies assemble into clusters.

The researchers also speculate that two distinct kinds of dark matter may exist-one that congregates around individual members of a cluster of visible galaxies and another that gathers around the cluster as a whole.

Yasushi Ikebe of the University of Tokyo and his colleagues report their findings in the Feb. 1 Nature.

To trace the dark matter in a cluster of galaxies, Ikebe and his colleagues measured the distribution of hot, X-ray-emitting gas that resides there. They made the standard assumption that the pressure exerted by the hot gas equals the gravitational attraction of the cluster. Under this condition, regions that have a higher density of dark matter will trap more of the gas.

In July 1993 and January 1994, Ikebe's team used the Japanese X-ray satellite ASCA to map the X-ray-emitting gas within a nearby cluster, Fornax. Instead of being distributed smoothly within and just outside the cluster, the gas clumped into two distinct regions. Some of the gas gathered around NGC 1399, a massive galaxy at the center of Fornax, while a larger amount concentrated around the entire cluster.

From these findings, the researchers conclude that dark matter congregates into lumps on both the galactic scale and the much larger cluster scale. …

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