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.
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.
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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