ALBUQUERQUE, NEW MEXICO--The Milky Way galaxy is tearing apart its oldest inhabitants, and for the first time, astronomers are witnessing the slaughter. A striking image presented here 3 June at the meeting of the American Astronomical Society shows a globular cluster known as Palomar 5 being torn asunder by tidal forces of our home galaxy. Researchers say the observation, and others like it, could shed new light on the distribution of dark matter in the halo of the Milky Way.
Globular clusters are large spherical aggregations of extremely old stars. They probably formed when our galaxy did, some 12 billion to 14 billion years ago. About 150 globular clusters are known to loop in wide orbits around the Milky Way's center. Astronomers suspected that tidal forces, which arise because the Milky Way's gravity is stronger on one side of the cluster than on the other, could break up clusters--especially more diffuse ones.
Now there's definitive proof in data from the Sloan Digital Sky Survey, a large international project to map one-quarter of the sky in exquisite detail. On both sides of Palomar 5, a team led by astronomer Eva Grebel of the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, found long streams of stars, spanning a total length of 13,000 light-years, or 20 times the apparent width of the full moon on the sky. These "tidal tails" form when stars are torn loose from the cluster and then slowly drift away. "It's a very amazing structure," says team member Michael Odenkirchen, also at MPIA. "Nothing like this has ever been seen before."
To discover the tails, astronomers had to carefully filter out the countless stars and background galaxies in the field of view that did not match the expected colors and brightnesses of globular cluster members. The finding suggests that many other sparse globular clusters have been ripped to pieces in the past; detecting Palomar 5 in the process is apparently just a lucky catch. Grebel and her colleagues hope to find the "orphan" tidal tails of earlier disruption events by analyzing other Sloan data.
"This is a very exciting and beautiful result," says David Spergel, a theorist at Princeton University. Because the tails more or less delineate the orbit of the parent globular cluster around the Milky Way center--information that's impossible to come by otherwise--the data will help scientists to map the distribution of dark matter in the Milky Way, he says. "[Observations like these] can rule out or constrain dark matter models."