Hubble Snaps Some Moving MACHOs

Astronomers have photographed what could be MACHOs (massive compact halo objects)--hypothetical dark objects whose gravity could help explain the motions of the galaxy's visible stars. The five faint objects appear to be extremely dim stars; if they aren't a fluke, "it would be a fair statement to say that at least part of the dark matter mystery has been solved," says team member Harvey Richer of the University of British Columbia in Vancouver, Canada.

By comparing two Hubble Space Telescope images, a team led by Rodrigo Ibata of the European Southern Observatory (ESO) in Munich, Germany, found the five objects moving very slightly in the halo of old stars that surrounds our galaxy. The astronomers first looked for faint traces of MACHOs in a 1995 Hubble image called Deep Field North. A single image could not reveal whether any of the objects were MACHOs, but in a second image, any object orbiting in the halo of the galaxy would probably betray itself by moving across the sky. In a paper to appear later this year in Astrophysical Journal Letters, Ibata and his colleagues list five faint, bluish objects that changed position between December 1995 and December 1997.

He and his colleagues suspect the objects are old, dim white dwarfs a few thousand light-years from Earth. Although it is hard to extrapolate accurately from such a small population, if this number of white dwarfs were scaled up to the whole of the galaxy, the total would be on the order of a few trillion--in good agreement with the results of indirect MACHO searches, which track the flickering of distant stars as dark objects move across the line of sight. Definitive confirmation may come from wide-angle surveys with large ground-based telescopes. Richer and his colleagues have their fingers crossed: "If the ground-based surveys don't find them, our scenario is not correct," he says.

Other astronomers caution that the scenario would create problems of its own for theorists. White dwarfs originate when aging stars shed their outer layers, which contain ash from stellar burning--elements heavier than hydrogen and helium. Their formation in vast numbers would leave the universe highly enriched in these elements, contrary to observations. And Ken Freeman of Mount Stromlo Observatory in Canberra, Australia, cautions: "It is not clear at this stage exactly what the MACHO objects are, so I am not sure if this is the first time that MACHOs have been imaged directly."