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Sloan Digital Sky Survey probes dark halos of galaxies

A new study using the Sloan Digital Sky Survey telescope in New Mexico provides the most direct evidence yet that galaxies reside at the center of dark matter concentrations that may be 50 times larger than the visible galaxy itself.

dy directly supports the generally accepted astronomical theories of dark matter and contradicts an alternative theory of gravity known as Modified Newtonian Dynamics (MOND), said New Mexico State University astronomy professor Anatoly Klypin.

Klypin and his colleagues, including Francisco Prada of the Max Planck Institute for Astronomy in Germany and the Instituto de Astrofisica de Canarias in Spain, studied the motion of about 3,000 satellites orbiting isolated bright galaxies and found strong evidence of dark matter's gravitational effects. They will present their findings at an international astronomy conference in Spain in late May.

The study "is important because it is a direct measurement of some of the properties predicted for dark matter," Klypin said.

Although it cannot be observed directly, dark matter is believed to account for about 27 percent of the total mass of the universe, compared with only about 3 percent for normal, observable matter. The rest, according to standard models of the structure and evolution of the universe, consists of dark energy and radiation.

The researchers observed 250,000 galaxies using the Sloan Digital Sky Survey (SDSS) telescope, located at Apache Point Observatory in the Sacramento Mountains of southern New Mexico, to find good candidates for a study of the gravitational effects of dark matter. From this data they identified about 3,000 satellites -- generally small galaxies orbiting large ones -- for which they could measure velocities.

The velocity of a satellite declines predictably as the satellite moves away from the object it is orbiting, due to the effects of gravity, Klypin said. In the case of planets in our solar system, where there is too little dark matter to have a gravitational effect, the decline is rapid because there is no mass between the planets and the sun, he said. But in the outer reaches of galaxies, where dark matter is believed to be clustered, the decline in velocity would be much more gradual if standard cosmological models are correct.

"Our results are consistent with dark matter," Klypin said. The findings provide strong evidence against the MOND alternative theory of gravity, he said. MOND, controversial since it was proposed in 1983, eliminates the need for dark matter in explaining the nature of the universe, by changing the law of gravity in areas such as the outskirts of galaxies.

The researchers said the study utilized the unique capabilities of the SDSS to probe dark matter in a way that cannot be done by any other current experiment.

They determined the velocity of each satellite relative to the galaxy it orbits by measuring the redshift in the objects' light spectra. Redshift, a change in light waves as an object moves away from an observer, is a standard means of measuring the speed of a distant object.

The next stage of the research will be to extend the sample of galaxies and satellites studied to improve the accuracy of the results.

NMSU astronomy professor Jon Holtzman and graduate student Mayrita Vitvitska also participated in the study, which will be presented to a joint conference of the Isaac Newton Group of Telescopes and the Instituto de Astrofisica de Canarias May 26-30 in La Palma, Canary Islands, Spain.

Prada is the lead author of the research paper. Other authors, in addition to Klypin, Holtzman and Vitviska, are David J. Schlegel of Princeton University, Eva K. Grebel and Hans-Walter Rix of the Max Planck Institute of Astronomy in Germany, Jonathan Brinkmann of the Apache Point Observatory staff, T.A. McKay of the University of Michigan and I. Csabai of Eotvos University in Budapest, Hungary, and the Johns Hopkins University in Baltimore.

New Mexico State University is a member of the Sloan Digital Sky Survey and also operates the Apache Point Observatory for the Astrophysical Research Consortium.

SDSS, considered the most ambitious astronomical survey project ever undertaken, will map in detail one-quarter of the entire sky, determining the positions and absolute brightnesses of more than 100 million celestial objects. It will also measure the distances to more than a million galaxies and quasars.