Seventy-Year Quest For Galactic Dark Matter Ends With Discovery Of Population Of Cool White Dwarfs In The Halo Of Our Galaxy

Scanning digitized images of the southern sky, the team found 38 previously unseen cool white dwarfs within about 450 light years of Earth. If the density of these newly discovered white dwarfs — apparently members of the galactic halo zooming through the disk of the galaxy — is indicative of the rest of the galaxy's halo, these dead stars would comprise at least three percent of the dark matter of the halo, and perhaps as much as 35 percent.

The discovery provides at least a partial answer to a question that has bedeviled astronomers for years: What is the identity of the missing mass that keeps galaxies from flying apart and the universe from expanding faster than it does?

"We've found a previously undetected population of stars in the galactic halo that represents a fraction of the baryonic dark matter in the galaxy," said lead author Ben R. Oppenheimer, a Hubble post-doctoral research fellow at the University of California, Berkeley. "This raises a lot of questions about our understanding of the star formation history of the galaxy and the basic processes of star formation."

Oppenheimer and colleagues Nigel C. Hambly and Andrew P. Digby of the Institute for Astronomy at the University of Edinburgh in Scotland; Simon T. Hodgkin of the Institute of Astronomy at Cambridge University in England; and theorist Didier Saumon of Vanderbilt University in Nashville, Tenn., will publish their results in the March 23 issue of Science.

Dark matter has puzzled astronomers since the observation in 1933 by Fritz Zwicky that clusters of galaxies don't contain enough visible stars to explain their rotation. The same is true of individual galaxies, and today, astronomers estimate that about 95 percent of the mass of galaxies like our own is too dark for astronomers to see. Much of this unseen mass resides in the spherical halo that envelops the disk of our galaxy.

Observations indicate that at most 35 percent of the galactic dark matter is made up of normal, so-called baryonic, matter — the stuff of stars and humans, composed of protons, neutrons and electrons. The rest is predicted to be massive particles that interact weakly with the rest of matter — a veritable zoo of possible creatures, from heavy neutrinos, called neutralinos, to photinos or axions. These are lumped together under the rubric weakly interacting massive particles, or WIMPs. Normal baryonic matter is assumed mostly to be in the form of massive compact halo objects, or MACHOs.

Despite intense efforts to find MACHOs or WIMPs, no one has directly detected either. In 1995, the MACHO project, led by Charles Alcock, now a professor of physics and astronomy at the University of Pennsylvania, indirectly detected MACHOs by measuring the effect of their presence on the images of distant stars. They concluded that these MACHOs comprise between eight and 50 percent of the mass of the halo. They also found that the individual MACHOs must have masses similar to those of white dwarfs.

"These indirect observations indicated that white dwarfs may make up a substantial faction of the dark matter, but the results could have been, and were, interpreted differently by some astronomers," Oppenheimer said. "They knew compact objects were in the halo, and they could measure the masses, but they never saw the objects themselves."

The new population of white dwarfs in the halo "provides a natural explanation for the microlensing results," the team wrote in its paper.

Oppenheimer and his colleagues found dark matter in the halo of our own galaxy by looking for extremely cool white dwarfs. White dwarfs are Earth-sized objects with about half the mass of the Sun, the core of what once was a red giant star that expelled large fractions of its mass into space. About 94 percent of all stars end up as white dwarfs, mere cooling cinders because they lack the fuel necessary for further nuclear fusion. Only recently did theorists realize that the oldest and coolest white dwarfs are, in fact, blue.

Until three years ago, most people assumed that as white dwarfs age, cool and fade away, they get redder and redder. Then, Brad M. S. Hansen of Princeton University and Saumon of Vanderbilt independently predicted that white dwarfs should get bluer rather than redder as they cool below about 4,500 Kelvin (about 8,000 degrees Fahrenheit). Apparently hydrogen molecules form at such low temperatures and, when compressed in the 10-meter-thick atmosphere of a white dwarf, absorb infrared light from the surface and re-emit it as blue light.

Oppenheimer and colleagues confirmed this theoretical prediction last year when they measured the spectrum of a bluish white dwarf Hambly and Hodgkin had discovered in 1997 in the halo of our galaxy, the Milky Way. A group led by Rodrigo Ibata of Strasbourg University also recently found three blue cool white dwarfs in the halo.

To determine just how many there might be in the halo, Hambly searched through photographic plates of the southern sky taken over the past 30 years, which he had recently digitized as part of the Royal Observatory Edinburgh's plate scanning project. From plates covering 196 sky fields around the south galactic pole — an area equal to 10 percent of the sky — Hambly picked out 92 objects that looked like nearby cool white dwarfs because they had moved slightly over the years. Oppenheimer, Hambly and Digby last October took spectra of the objects with a four-meter telescope at the Cerro Tololo Interamerican Observatory in Chile.

Of the 69 stars for which they were able to obtain spectra, 38 turned out to be white dwarfs, and 34 of those were very cool, many exhibiting the bluing effect.

This small set of barely visible stars may, however, be the tip of an iceberg representing a vast population of cool white dwarfs too faint to see and comprising as much as 35 percent of the halo dark matter, he said. This population of white dwarfs would be a relic of the earliest days of the galaxy, between 10 and 13 billion years ago.

"This research is not just about white dwarfs and dark matter. It also has implications for the star formation history of the galaxy, probably even before the disk itself formed," Saumon said. "These cool white dwarfs are the fossils of the early population of halo stars. There is much to learn about how galaxies form, and about how stars form in the process, from studying this population of white dwarfs. So this will really branch out into many different areas of astrophysics. There's the dark matter. There's the history of the galaxy. All that's tied to this discovery."

Theories of galaxy and star formation will have to explain why the halo stars are overwhelmingly white dwarfs while the lower mass stars, which have not had enough time to evolve into white dwarfs, seem to be missing or are in abnormally small numbers.

"If this is true and our galaxy is not weird, by looking at distant and younger galaxies we may be able to see this epoch of galaxy evolution," Oppenheimer said.

Oppenheimer and his colleagues hope to look for similar cool white dwarfs around the northern galactic pole, and to study more closely the new-found white dwarfs and their peculiar bluing effect to better understand their atmospheres.

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A document containing answers to some questions about this work can be found at http://astron.berkeley.edu/~bro/faq.html. For pictures of a bluish, cool white dwarf, go to http://www.berkeley.edu/news/media/download/.