Three "rivers" of stars have been discovered stretching thousands of light years across the northern sky. The smoothness of their trailing arcs may help scientists learn more about the nature of dark matter in our galaxy, the Milky Way.
Earlier in March, Caltech researcher Carl Grillmair (L) and Roberta Johnson, a graduate student at California State University, Long Beach, both in the US, reported a tail of stars covering at least 45° of the sky. It lies between the Big Dipper’s bowl and Arcturus, a bright star in the constellation Bootes.
The tail is about 76,000 light years away and is thought to consist of thousands of stars travelling ahead of and behind the cluster NGC 5466. "Since then, I’ve found two more, even longer ones," Grillmair said. "It turns out they’re all over the place."
None of these three had been observed before. The tails are far too distant to be seen with the naked human eye and their light is normally obscured by the huge number of Milky Way stars that sit between them and Earth.
Grillmair and Johnson discovered the arcs by analysing data released in June 2005 by the Sloan Digital Sky Survey. The data related specifically to stars in a portion of the sky known as the north galactic cap.
The Sloan survey was designed to help cosmologists better understand extremely distant objects, far beyond our own galaxy, and to verify models of how the universe formed. Grillmair says they are working with "the detritus of the sky survey" – data on the stars in the Milky Way.
To locate the streams, the researchers looked at the brightness and colour of millions of stars in the survey. Stars that originate in the same cluster are typically thought to be born at the same time and under the same conditions. So by looking for similar distributions of star brightness and colour, the pair identified stars that had been stripped away from the outer regions of a parent cluster.
Grillmair says the final proof that the tails emanate from clusters will come when astronomers get good measurements of the stars’ velocities. "If they’re just random field stars, they should have velocities hundreds of miles per second different from each other," he says. But if they were tidally (gravitationally) torn away from the same cluster, their velocities should be the same to within a couple of miles per second, he says.
The Sloan survey did not gather data on the entire stretch of NGC 5466’s tail. So Grillmair says the tail may be much longer. And he says the survey only measured the brightest stars in the stream – as many as five times more stars could simply be too faint to be seen.
Beyond identifying previously unknown features in the Milky Way, the streams may also help astronomers study the galaxy’s distribution of dark matter – the hypothetical invisible material thought to make up a large part of matter in the universe. They preserve a record of their clusters’ journeys and gravitational interactions around the Milky Way in orbits that last hundreds of millions of years.
Grillmair says if the streams had been affected by the haloes of dark matter some astronomers think exist, then they would appear more "serpentine". But he says none of the tails observed so far show any kinks, suggesting "whatever is out there is very smoothly distributed". He cautions that the finding only applies to the areas where these clusters have orbited and says nothing about the possibility of such haloes elsewhere in the galaxy or universe.