"Lost" Dark Matter Discovered in Space, Scientists Say

John Roach
for National Geographic News
September 28, 2005
Astronomers say they have found a type of matter that cannot be seen, but which is thought to dominate the cosmos, in a place where it was thought not to exist.

The finding gives weight to a common theory of how the universe is pieced together.

The exact nature of the matter, called dark matter, is unknown, but scientists believe it accounts for more than 90 percent of the mass in the universe.

Though invisible, scientists think dark matter exists because stars appear to be accelerated by the gravity of a mass greater than all of the visible stars and dust in space.

But this theory was challenged in 2003 when a team of astronomers reported that certain stars in one type of galaxy actually move slowly, suggesting an absence of dark matter.

The new study, based on computer simulations, explains this seemingly odd behavior.

The explanation fits with the theory that these galaxies, like all galaxies, are embedded in haloes of dark matter, said Avishai Dekel, a physics professor at the Hebrew University of Jerusalem in Israel.

"It seems that we have identified the main solution to the problem," Dekel wrote via e-mail.

Dekel is the lead author of the study appearing in tomorrow's issue of the journal Nature.

Galaxy Matters

The theory that galaxies are embedded in dark matter stems from the observation of stars in spiral galaxies. These galaxies, including the Milky Way, are flattened spiral-shaped disks of stars and gas.

Since most of the visible matter in a spiral galaxy is concentrated at the center, stars there would be expected to move more quickly than stars on the outskirts of the galaxy, according to the laws of gravity.

But observations show that the stars in the outskirts of spiral galaxies move just as quickly as those closer to the center. Scientists explain this phenomenon by the gravitational influence of dark matter in and around the galaxy.

But in elliptical galaxies, which are round, smooth collections of stars, it is difficult to study the motion of stars on the outskirts.

In the 2003 study, scientists found that stars in these galaxies move more slowly on the outskirts. The researchers suggested this was evidence for a lack of dark matter in elliptical galaxies.

Since at least some elliptical galaxies are believed to form from the mergers of spiral galaxies, the lack of dark matter in elliptical galaxies confounded the scientific community.

"Where did the dark matter disappear to during the merger?" Dekel said.

Computer Simulations

To find out, the team behind the new study ran computer simulations of galaxy mergers and analyzed the simulations.

The researchers, based in the U.S. and France, found that during galactic mergers, some stars are thrown into elongated orbits that run perpendicular to a galaxy's center.

These stars are the easiest for researchers to study in elliptical galaxies. To explain why, Dekel said to imagine viewing from across the room a ball of needles pointing out from a central point. The needle tips that are easiest to see are those that are perpendicular to your line of sight, sticking straight up, down, left, or right.

But scientists can only measure the speed of objects that are moving to or away from them, not those moving perpendicular to their line of sight.

So Dekel and his colleagues concluded that the slow-moving stars tracked in the 2003 study were actually moving in these elongated orbits, perpendicular to the astronomers' line of sight.

Those stars may have actually been moving at a high velocity without much motion toward or away from the viewer, thus taking on a slow-moving appearance, Dekel explained.

Study Response

Michael Merrifield, an astrophysicist at the University of Nottingham in England, co-authored the 2003 study suggesting that elliptical galaxies lack dark matter.

"The jury is still out" on the presence of dark matter in elliptical galaxies, he said.

Computer modeling done by his team did not generate the scenario envisioned by Dekel and colleagues, he said.

However, the 2003 study authors did find some dark matter in elliptical galaxies, according to Aaron Romanowsky, lead author of the paper, who now works at the University of Concepción in Chile.

"We still think [the dark matter] is on the weak side," Romanowsky wrote in an e-mail interview. "One difference between our models is that theirs already has a lot of dark matter in the galaxy centers, which seems incorrect based on what we know about galaxies so far."

Both teams agree that astronomers are making progress toward resolving the issue, which is essential to understanding the formation and structure of galaxies.

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