Published online 31 May 2007 | Nature | doi:10.1038/news070528-7

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Space telescope spies dark matter

Triangulation technique spots object at the edge of our Galaxy.

By viewing an object at the edge of the Galaxy from two points 70 million miles apart, you can pinpoint its position.By viewing an object at the edge of the Galaxy from two points 70 million miles apart, you can pinpoint its position.NASA/JPL-Caltech/T. Pyle (SSC)

Astronomers have used telescopes on Earth and in space to nail the precise position of a mysterious, dark object at the outer edge of our Galaxy. The work could be an important step in understanding so-called dark matter — mysterious material that makes up about a quarter of our Universe.

Most dark matter is believed to be in the form of subatomic particles that don't interact with regular atoms. But as much as 20% could also be in more traditional things that don't emit light, such as black holes and clouds of gas that never became stars.

Such objects are thought to litter the galactic halo — the region beyond the visible disk of the Galaxy. They are known as Massive Compact Halo Objects, or MACHOs.

MACHOs can be spotted if they pass in front of a distant star. The object's gravitational field will bend the starlight like a lens, briefly making the star appear slightly brighter.

But a single observation tells you only that the MACHO is between you and the star, not how far away it is. Two viewpoints allows distance to be calculated, similar to the way that binocular vision gives us depth perception.

Double vision

In the summer of 2005, Subo Dong, an astronomer at Ohio State University in Columbus and his colleagues watched from multiple ground-based telescopes as a distant MACHO caused a star in the Small Magellanic Cloud, a nearby galaxy, to briefly brighten.

Half a day later, they watched the brightening again with the Spitzer Space Telescope, an infrared telescope orbiting 70 million kilometres from Earth. The delay corresponded to the time it took the MACHO to cross the space between the lines of sight from Earth and the space telescope (see diagram).

Based on this delay, the team calculated that the object lies some 16,000 light years away, putting it squarely in the Milky Way's halo. Dong suspects that the object is a pair of relatively small black holes orbiting each other.

"I think it's great," says cosmologist and Nobel laureate John Mather of the Goddard Space Flight Center in Greenbelt, Maryland. Astrophysicists have long talked about making such measurements, but this is the first time they've done so successfully, Mather says.

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Dong cautions that there is a small chance that the effect the telescope saw was actually in the distant Magellanic Galaxy. The Spitzer will need to spot more such objects before astronomers can be sure where the MACHOs are.

Subo and the team presented their results on 30 May at the American Astronomical Society's meeting in Honolulu, Hawaii, and will publish them in an upcoming issue of the Astrophysical Journal.

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