Dark matter: MACHOs in Milky Way's halo?

Two independent teams of astronomers report new evidence this week that dark matter resides at the outskirts of the Milky Way. Dark matter -- invisible material thought to lie at the periphery of many galazies -- doesn't glow like ordinary matter, yet exerts a gravitational tug.

The studies, based on nightly scans of millions of stars in the nearby Large Magellanic Cloud (LMC) galaxy, suggest that the Milky Way's periphery contains dense pieces of dark matter called Massive Compact Halo Objects, or MACHOs. The new findings rely on the phenomenon of gravitational lensing, in which a massive foreground object bends and brightens light from an object that lies behind it.

After several years of searching, the astronomers report that three stars in the LMC have shown such telltale brightening. The researchers attribute all three increases to gravitational lensing by Milky Way MACHOs that happened to cross the line of sight between the stars and Earth. Each MACHO may have a mass roughly one-tenth that of the sun and a size about that of a small star.

A U.S.-Australian team found evidence for a single MACHO, based on observations of an LMC star that suddenly increased its brightness nearly sevenfold. Another group, at the Centre d'Etudes de Saclay in Gif-sur-Yvette, France, found evidence for two MACHOs. This team bases its conclusions on separate observations of two LMC stars, each of which briefly appeared about three times as bright as usual. The two groups say the brightening is probably not caused by variations in the luminosity of the stars.

"For me, these three [observations] show that dark matter exists," says Sylvain Zylberajch of the Saclay group, which looks for MACHOs using two telescopes at the European Southern Observatory in La Serena, Chile. The two teams presented their data in Italy at the International Conference on the Cosmic Microwave Background Radiation in Capri and the Gran Sasso Conference on Underground Particle Physics.

The U.S.-Australian team led by Charles Alcock of the Lawrence Livermore (Calif.) National Laboratory, has looked for MACHOs since 1992 using an elderly but refurbished 1.6-meter telscope at the Mount Stromlo Observatory near Canberra, Australia. Using a computer to analyze changes in light emission from 1.8 million stars in the LMC, the team discovered two weeks ago that one -- perhaps a red giant -- had brightened for 33 days early this year, says team member Kim Griest of the University of California, San Diego.

The symmetrical rise and fall in brightness suggested the handiwork of a gravitational lens, he says. In addition, the increase appeared the same in both red and blue light, another indication that the star had not suddenly boosted its own light output. Nonetheless, notes Griest, his group wasn't yet ready to go public with the finding. But after learning that the French team was about to announce its own recent discoveries, the researchers followed suit.

Contacted in Capri where he heard the U.S.-Australian report, Richard Saunders of the Military Radio Astronomy Observatory in Cambridge, England, said he was skeptical at first. "I though, [the brightening] was just a background quasar. Then [the presenter] showed his light curve....I was very impressed."