Halo of the Milky Way: not so MACHO?

The halo of our galaxy just got stranger.

Evidence has mounted since the 1970s that the halo -- the envelope of material that extends to the outskirts of the Milky Way -- contains lots of dark matter. This invisible material must provide the gravitational glue that holds the rapidly rotating outer regions of the galaxy together. Its visible stars and glowing gas simply don't have enough mass to keep the Milky Way intact.

Now, using a technique that takes advantage of one of the odder properties of gravity, scientists report that no more than 20 percent of that dark matter consists of objects ranging from small black holes to dim, planetlike bodies similar in mass to Jupiter.

Collectively known as MACHOs (massive compact halo objects), these dark matter objects are thought to be composed of baryons -- neutrons, protons, and other building blocks of ordinary matter. The new study suggests, however, that the halo consists mostly of exotic material totally unlike ordinary atoms.

Kem H. Cook of Lawrence Livermore (Calif.) National Laboratory and his colleagues presented these and other findings last week at a meeting of the American Physical Society in Washington, D.C., and in the April 10 Physical Review Letters.

"Combining our results for the dark halo and the galactic center is forcing us to reevaluate the standard model of our galaxy's structure, as well as its dark halo," Cook says. To search for dark matter, the team relies on gravitational lensing. In this phenomenon, a massive object, whether visible or not, betrays its presence by bending and brightening the light emitted by a body behind it.

Using a telescope at Mount Stromlo Observatory near Canberra, Australia, the team scans millions of stars in the central bulge of our galaxy and in its neighbor, the Large Magellanic Cloud (LMC).

Viewing the center of our galaxy, the Mount Stromlo team and another group found many more lensing events than expected. Because dark matter doesn't concentrate at galactic centers, scientists suggest that visible stars cause the lensing. Cook says the finding confirms that our galaxy's bulge has a bar-shaped concentration of stars.

To view the LMC, the telescope's line of sight must pass through the Milky Way's halo. So far, the Mount Strombo team has found the fingerprints of four MACHOs. That represents only about one-fifth the mass needed to account for all the dark matter they expected to find in that area of the halo, Cook notes.

MACHOs might contribute even less if researchers have misidentified a true increase in the brightness of an LMC star as a lensing event, says collaborator Kim Griest of the University of California, San Diego.

If the halo's dark matter isn't mostly MACHO, what then? David N. Spergel of Princeton University suggests that, like the rest of the cosmos, the Milky Way's outskirts may contain a melange of exotic dark matter particles with a sprinkling of "ordinary" dark matter.