For the last 70 years, as astronomers have gazed up into the heavens, they have known there are two types of matter in the universe. There is the matter that glows, like stars and clouds of heated gas, that they can see and there is matter that is invisible, that cannot be seen but must exist because it exerts a detectable gravitational influence on the visible matter.
This invisible matter is sometimes called dark matter and sometimes referred to as the missing mass. Some experts argue that it is simply ordinary matter (what scientists call baryonic matter) that is too cold to glow. Others maintain that not enough ordinary matter was made when the universe was created to account for all the dark matter that must exist so it must be made of something far more exotic. Now, a careful star survey conducted by an international team of astronomers, including Didier Saumon from Vanderbilt University, has concluded that a significant portion of the dark matter that forms an invisible halo around the Milky Way galaxy is made up of something very prosaic: dead stars, dim celestial objects called cool white dwarfs.
Writing in the March 23 issue of the journal Science, the team reports that they found 38 previously unseen cool white dwarfs within about 450 light years of Earth in a careful survey of a small portion of the southern sky. They calculate that if these dim stellar remnants are spread evenly throughout the galactic halo then they can account for at least 3 percent, and perhaps as much as 35 percent, of the dark matter in the galaxy.
If their analysis is correct, the galactic halo is filled with a vast population of these dim, dying stars, many of which must be between 10 billion and 13 billion years old, making them relics of the earliest days of the galaxy. If enough of these stellar remnants can be found and characterized, it should provide scientists with important new insights into the formation of the galaxy.
“This research is not just about white dwarfs and dark matter. It also has implications for the history of the galaxy, probably even before the disk itself formed,” Saumon said. “There is much to learn about how galaxies form, and about how stars form in the process, from studying this population of white dwarfs”
Co-authors of the paper with Oppenheimer, a Hubble post-doctoral research fellow at the University of California, Berkeley, and Saumon, an assistant professor of physics and astronomy at Vanderbilt, were Nigel C. Hambly and Andrew P. Digby of the Institute for Astronomy at the University of Edinburgh in Scotland; and Simon T. Hodgkin of the Institute of Astronomy at Cambridge University in England. Dark matter has puzzled astronomers since Fritz Zwicky’s observation in 1933 that clusters of galaxies don’t contain enough visible stars to explain their rotation. The same holds 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.
Previous studies have concluded that no more than 35 percent of the galactic dark matter is made up of normal, baryonic matter—the stuff of stars and humans, composed of protons, neutrons and electrons. Physicists predict that the rest is made from massive particles that are invisible because they only interact slightly with normal matter. They have proposed a veritable zoo of exotic dark matter particles, ranging from heavy neutrinos, called neutralinos, to photinos or axions. These are lumped together under the rubric of weakly interacting massive particles, or WIMPs. Normal baryonic matter is assumed to be mostly 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 Prof. Charles Alcock, now at the University of Pennsylvania, indirectly detected MACHOs. The astronomers used a technique called “microlensing” that measures the effect of massive but invisible objects by their effect on the images of distant stars. They concluded that MACHOs comprise between 8 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. But they never saw the objects themselves and other astronomers came up with alternative explanations for their measurements. “The direct observation of this new population of white dwarfs cuts through the controversy and provides a natural explanation for the microlensing results,” Saumon said.
The astronomers found dark matter in the halo of our own galaxy by looking for extremely cool white dwarfs. White dwarfs are the final stage in the lives of more than 99 percent of the stars in the galaxy. Typical stars, like the Sun, burn steadily for billions of years before running out of nuclear fuel. When that occurs, they cool and swell tremendously in size to become red giant stars, like the star Betelgeuse. This stage lasts for about 10 percent of a star’s lifetime. Next the bloated star blows off its outer layers to form a planetary nebula. The star’s core condenses down to an Earth-sized object with 60 percent of the mass of the Sun. When first created, a white dwarf burns at temperatures that are actually hotter than those of the original star. But, over billions of years, the dwarf star gradually cools and fades.
Until three years ago most people assumed that as white dwarfs age, they get redder and redder. Then Saumon and Brad M. S. Hansen of Princeton University independently predicted that one type of white dwarfs, those that are hydrogen-rich, should actually turn bluer as they cool below 4,500 degrees. At about that temperature hydrogen molecules form a 10-meter-thick atmosphere around the dwarf star. The gravity is so strong that the molecules are compressed to the point where they absorb much of the infrared light coming from below. This prediction was confirmed last year by Oppenheimer’s team and another group led by Rodrigo Ibata of Strasbourg University.
Saumon has developed two models for cool white dwarfs: one with outer layers of pure hydrogen and the other with outer layers of pure helium. The simulations allow him to predict the spectra of these objects, information that was instrumental in characterizing the new dwarf stars that the group discovered.
To estimate the number of cool white dwarfs 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 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 measured the spectra of these 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 from the galactic halo.
Next, Oppenheimer, Saumon and their colleagues intend to look for similar cool white dwarfs around the northern galactic pole, and to study more closely the white dwarfs and their peculiar bluing effect to better understand their atmospheres.