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Sloan Digital Sky Survey

CONTACTS:

Dr. Francisco Prada
Instituto de Astrofisica de Canarias, Spain
+34-922425431, fprada{at}ing.iac.es

Dr. Anatoly Klypin
New Mexico State University, Las Cruces
(505) 646-1400, aklypin{at}nmsu.edu

Gary S. Ruderman
Public Information Officer, Sloan Digital Sky Survey
312-320-4794, sdsspio{at}aol.com

May 21, 2003

Sloan Digital Sky Survey Probes Dark Matter Theory

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

The study very directly supports the generally accepted astronomical 
theories on dark matter and contradicts an alternative theory of
gravity known as Modified Newtonian Dynamics (MOND).

Lead investigator, Francisco Prada, of the Max Planck Institute for 
Astronomy in Germany and the Instituto de Astrofisica de Canarias in 
Spain, and a team of colleagues from New Mexico State University at
Las Cruces and at other Sloan Digital Sky Survey (SDSS) institutions,
will present the results of the team's research at 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 and his colleagues studied the motion of about 3,000 satellites 
orbiting isolated bright galaxies and found strong evidence of dark 
matter gravitational effects.

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

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.

Prada and his colleagues observed 250,000 galaxies from the Sloan 
Digital Sky Survey (SDSS) telescope to find good candidates for a
study of the gravitational effects of dark matter. From the SDDS
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. 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 imply the presence of dark matter," Prada 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.

Researchers said the SDSS study probes dark matter in a way that
cannot be done by any other current experiment. This type of research
could not have been done without the unique capabilities of the SDSS.

The researchers determined the velocity of each satellite relative to 
the galaxy it orbits by measuring the redshift in the light spectra
from the objects. 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. 

Participating institutions in this discovery are: The Max Planck 
Institute for Astronomy, Germany; Centro Astronomico Hispano-Aleman;
New Mexico State University; Princeton University Observatory; Apache
Point Observatory; University of Michigan; Eötvös University,
Budapest; and Johns Hopkins University.

Authors of the paper are:

* Francisco Prada, Max Planck Institute for Astronomy, Konigstuhl 17, 
D-679117, Heidelberg, Germany; Centro Astronomico Hispano-Aleman,
Apdo. 511, E-04080, Almeria, Spain; Instituto de Astrofisica de
Canarias, E-38205, Tenerife, Spain; The Isaac Newton group of
Telescopes, Apdo. 321, E-38700, La Palma, Spain, fprada{at}ing.iac.es
* Mayrita Vitviska, Astronomy Department, New Mexico State
University, Box 30001, Department 4500, Las Cruces, NM 88003,
mvitvit{at}nmsu.edu * Anatoly Klypin, New Mexico State University,
aklypin{at}nmsu.edu * Jon A. Holtzman, New Mexico State University,
holtz{at}nmsu.edu * David J. Schlegel, Princeton University Observatory,
Peyton Hall, Princeton, NJ 08544-1001, schlegel{at}astro.princeton.edu
* Eva K. Grebel, Max Planck Institute for Astronomy,
  grebel{at}mpia-hd.mpg.de
* Hans-Walter Rix, Max Planck Institute for Astronomy,
  rix{at}mpia-hd.mpg.de
* Jon Brinkman, Apache Point Observatory, P.O. Box 59, Sunspot, NM 
  88349, jb{at}apo.nmsu.edu
* Tim M. McKay, Department of Physics, University of Michigan, 500 E. 
  University Ave., Ann Arbor, MI 48109, tamckay{at}umich,edu
* Istvan Csabai, Department of Physics, Eötvös University, Budapest,
  Pf. 32, Hungary, H-1518, csabai{at}galahad.elte.hu

Additional information about the conference is at:
     http://www.iac.es/proyect/sattail/

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