Star velocities defy expectations based on standard theory
Very distant galaxies have surprisingly little dark matter, the invisible stuff thought to make up the bulk of matter in the universe, new observations suggest.
Stars in the outer regions of some far-off galaxies move more slowly than stars closer to the center, indicating a lack of dark matter, astronomer Reinhard Genzel and colleagues report online March 15 in Nature. If confirmed, the result could lead astronomers to reconsider the role dark matter played in early galaxy evolution and might also offer clues to how nearby elliptical galaxies evolved.
In contrast with these distant galaxies, stars orbiting on the outskirts of the Milky Way and other nearby galaxies move too fast for their velocities to result only from the gravity of gas and stars closer to the galactic center. If visible galactic matter is embedded in a cloud of invisible dark matter, though, gravity from the invisible matter can explain the high stellar velocities. Using stars’ orbital velocities in nearby galaxies as a reference, astronomers expected that stars in galaxies farther away would behave similarly. “Turns out that is not the case,” says study coauthor Stijn Wuyts of the University of Bath in England.
The plot of stars’ velocities relative to their distances from a galaxy’s center is called a galactic rotation curve. In the new paper (and others posted online at arXiv.org), the team presents rotation curves measured with the Very Large Telescope in Chile for more than 100 distant galaxies, some seen as they were just a few billion years after the Big Bang, 13.8 billion years ago.
When the velocities of stars diminish as the stars get farther from their galactic centers, that rotation curve is said to fall off with distance. “The data provide a strong indication that falling curves are common in this population of early galaxies,” says Genzel, of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.
In the papers, the authors provide several plausible explanations for the falling rotation curves, but it’s not clear which is correct, says UCLA astronomer Alice Shapley, who was not involved in the study. The results “reveal that we don't fully understand all of the details of how [galactic] disks form in the early universe.”
Genzel and colleagues suggest that the rotation curves fall off because turbulent streams of gas bring more and more material into inner regions of these distant galaxies. The gas piles up there, while dark matter is left on the outskirts. Explosions of stars and winds flowing from black holes might also drag dark matter away from the inner regions of the galaxy. Or dark matter may be distributed on a scale of hundreds of thousands of light-years, while the gas and stars in these early galaxies are interacting on a scale of only tens of thousands of light-years, Genzel says, so astronomers detect the gas but not the dark matter.
“Dark matter must be there,” Genzel says. “Without it, there are no galaxies and no ‘us,’ so we need to understand its nature and distribution to explain what we see in the universe.”
Current models of galaxy formation suggest that dark matter and ordinary matter intermixed in the early universe, so dark matter should tug on ordinary matter, resulting in rotation curves that are flat — not falling — for galaxies at the distance Genzel and colleagues observed. “We need to understand which physics is required to make simulations match the observations,” Shapley says.
Genzel notes that the way the gas and stars move in the distant galaxies is similar to the way gas and stars move in elliptical galaxies nearby. So, he suggests, the clumpy, irregular distant galaxies the team studied may be progenitors of elliptical galaxies in the local universe.
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