Survey unveils star cluster shredded by Milky Way
MAX PLANCK INSTITUTE FOR ASTRONOMY NEWS RELEASE
Posted: June 8, 2002
A team of astronomers from the Sloan Digital Sky Survey (SDSS) collaboration has discovered a spectacular stream of stellar debris emanating from a star cluster that is being torn apart by the Milky Way.
The stars in the newly discovered stream are being torn from an ancient globular cluster named Palomar 5, which is located in the outer part of our Galaxy 75,000 light years away from the Sun. While typical globular clusters are massive, luminous concentrations of some hundred thousand stars, Palomar 5 by comparison looks faint and diffuse and contains only about ten thousand stars. This led astronomers to suspect that Palomar 5 might be a likely victim of the disruptive tides of the Milky Way. These "tides" arise because the Milky Way's gravitational pull is stronger on the cluster's near side than on the far side, thus tearing the cluster apart. However, the telltale debris from the disruption was difficult to find since it is hidden in a sea of foreground and background objects.
Using data from the SDSS and a special filtering technique, Odenkirchen and his collaborators have succeeded in making the stream of debris from Palomar 5 directly visible. "The excellent homogeneity, resolution, depth, and multi-color information of the SDSS observations have allowed us to separate faint former members of Palomar 5 from contaminating field stars and background galaxies," says Odenkirchen, who is a postdoctoral researcher at the MPIA.
The SDSS is an international project that is creating a deep map of one quarter of the sky in five colors. The SDSS records objects up to 10 million times fainter than the faintest stars visible with the naked eye. The observations are carried out with a special wide-field camera on a dedicated 2.5-meter telescope at Apache Point Observatory, New Mexico. Team member Dr. Connie Rockosi of the University of Washington was one of the builders of the camera.
First direct evidence for the tidal disruption of Palomar 5 emerged two years ago from SDSS commissioning data that happened to include Palomar 5. Odenkirchen and collaborators were amazed to recover the characteristic S-shape signature of tidal debris from these data. "This was the first time that tidal tails of a star cluster were seen with convincing clarity", says Grebel, an astronomer who leads the Galactic structure group at MPIA.
Meanwhile the SDSS has scanned a much larger region on the sky. Analyzing the new data the researchers found that the two tails emanating from Palomar 5 extend over an arc of ten degrees on the sky. This vast area corresponds to 20 times the diameter of the full moon on the sky or to a length of 13,000 light years in space. "Remarkably, we now find more mass in the tails than in the remaining cluster. We expect to detect the stream over an even larger area as the survey progresses," Odenkirchen said.
Together with the so-called Sagittarius stream, which emerges from a dwarf galaxy that is currently being accreted by the Milky Way, there are now two different examples of extended stream-like structures in the Galactic halo. Computer simulations suggest that globular clusters were much more numerous in the early days of the Milky Way, and that many of them have already been shredded by Galactic tides. As the survey proceeds the SDSS researchers will be able to test this prediction by searching for signs of tidal mass loss around other globular clusters. "The SDSS data base will ultimately allow us to estimate the total number of such streams," says Professor Hans-Walter Rix, director of the MPIA. "This will clarify the role of tidal disruption in the build-up of the Galactic halo and provide a crucial test for galaxy formation models."
The researchers participating in this work are Michael Odenkirchen, Eva Grebel, Walter Dehnen, and Hans-Walter Rix from the Max Planck Institute for Astronomy, Connie Rockosi of the University of Washington, Brian Yanny from Fermilab, and Heidi Newberg from the Rensselaer Polytechnic Institute.
The SDSS is a joint project of The University of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, The Johns Hopkins University, Los Alamos National Laboratory, the Max-Planck Institute for Astronomy (MPIA), the Max-Planck Institute for Astrophysics (MPA), New Mexico State University, Princeton University, the United States Naval Observatory, the University of Pittsburgh, and the University of Washington.
Funding for the SDSS has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Aeronautics and Space Administration, the National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society.
DVD is here!|
The first in a series of space DVDs is now available from the Astronomy Now Store. Relive shuttle Columbia's March flight to refurbish the Hubble Space Telescope in spectacular DVD quality.
U.K. & WORLDWIDE STORE
The Apollo 14 Complete Downlink DVD set (5 discs) contains all the available television downlink footage from the Apollo 14 mission. A two-disc edited version is also available.