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Dim prospects. Invisible dark matter may have started out as hot white dwarfs like this one in the middle of planetary nebula NGC 2440.

Astronomers Glimpse Galaxy's Heavy Halo

For decades astronomers have been searching for the source of the excess gravity that twirls stars and gas around the fringes of galaxies. Now a team claims to have identified at least 3% of all such dark matter in the Milky Way, and possibly more. The unseen movers are fast-moving white dwarf stars that could account for as much as one-third of the galaxy's dark matter.

Most of the mass in a galaxy is invisible. Astronomers have deduced that astonishing conclusion from the following facts: The Milky Way's familiar pinwheel of relatively young stars sits amid an extended spherical halo of older stars and gas. Their combined gravity holds the galaxy together and keeps the stellar pinwheel spinning. Far enough from the center, the pull should eventually weaken and the stars slow down. But they don't. The best explanation is that almost 90% of the total mass of the galaxy is an invisible substance, spread throughout the halo, called dark matter.

The leading suspects have long been dim, cold, half-solar-mass stars called white dwarfs. They have the right mass, move fast enough, and should be common enough to account for the missing mass. Astronomers hadn't found enough of them to account for all the dark matter--but it turns out they were looking for the wrong color star. New theoretical models show that hydrogen in white dwarfs should emit blue light, not red as astronomers once thought.

This realization spurred a team led by astronomer Ben R. Oppenheimer of the University of California, Berkeley, to examine almost 200 digital images covering 12% of the sky. They found 126 fast-moving, faint objects and then tracked them for 4 nights from the 4-meter Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile. In the end, the team plucked out 38 new cool white dwarfs orbiting in the galactic halo. By multiplying the density of the newfound cool dwarfs by the volume of the galactic halo, Oppenheimer's team estimates that white dwarfs make up, by the most conservative estimate, at least 3% of the total galactic dark matter, they report online in Science on 23 March.

There is only one catch, astronomers say: Some of the newly discovered dwarfs might not actually be in the halo; instead, they are probably just passing through. A definitive answer should come within a year or two from several ongoing white dwarf surveys that are probing farther into the halo. "We are in the very early stages," says astrophysicist Harvey Richer of the University of British Columbia, Vancouver. "Things haven't all shaken out yet."

Related sites

Ben R. Oppenheimer's Web page

The white dwarf database