Dark vision.
NASA's Spitzer Space Telescope was used to determine the distance of a binary black hole.

Through a Lens, Darkly

HONOLULU, HAWAII--For the first time, astronomers have confirmed that invisible objects float around the outskirts of our galaxy. Using a form of cosmic triangulation, they have identified what appears to be a double black hole some 16,000 light-years away. The result, presented here today at the 210th meeting of the American Astronomical Society, is an important step in unraveling the nature of dark matter.

According to current theories, dark matter comes in two varieties. Most of it is expected to be in the form of yet-to-be-discovered elementary particles. But there must also be invisible matter consisting of normal atoms. Its nature is unknown, but theorists expect dim stars, free-floating planets, and black holes to lurk in the extended halo of the Milky Way. Trouble is, these "massive compact halo objects," or MACHOs, can't be seen directly.

But every now and then, a MACHO passes in front of a more distant star beyond the halo. The dark object's gravity amplifies the star's light, acting as a tiny gravitational lens. Astronomers have observed more than a dozen of these microlensing events over the past years in the Small Magellanic Cloud (a companion galaxy of the Milky Way). However, the temporary brightening of the distant star could also result when a much dimmer, ordinary star in the Magellanic Cloud itself passes in front of it. To find out whether the lensing object is indeed in the halo, you need to know its distance.

Using NASA's Spitzer Space Telescope, an international team of astronomers led by Andrew Gould of Ohio State University in Columbus has now carried out this distance determination for a microlensing event that occurred in the summer of 2005. Their method is comparable to the way surveyors estimate distances to mountaintops by taking measurements from two points. Spitzer orbits the sun some 40 million kilometers behind Earth. Therefore, it detected the microlensing event at a slightly different time than did astronomers on Earth. From this gap, the team deduced that the lens was in the Milky Way's halo, not in the more distant Magellanic Cloud. Therefore, the microlensing was due to a MACHO.

Further analysis of the observations also reveals that the lens consists of two bodies, weighing in at three and seven times the mass of the sun, respectively. Stars that massive would be visible, so the MACHO is probably a binary black hole, according to team member Subo Dong, also of Ohio State. Future observations will be needed to determine whether normal dark matter consists mainly of black holes. The team's results are accepted for publication in The Astrophysical Journal.

"People have been discussing this technique for a long time," says astrophysicist and Nobel laureate John Mather of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "It's great to see that they've finally succeeded in applying it." Charles Alcock, a MACHO researcher at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, adds that the result is important because the new technique allows astronomers to actually weigh MACHOs--something that cannot be done without knowing their distance.

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