Clustering of Quasars 10 Billion Light Years Away Determine Relationship with Dark Matter

(February 9, 2007) — Using a map of more than 4,000 luminous quasars in the distant universe, scientists from the Sloan Digital Sky Survey (SDSS-II) have shown that these brilliant beacons are strongly clumped, with huge quasar superclusters separated by vast stretches of empty space. The strong clustering shows that the quasars lie within massive concentrations of dark matter.

“Previous maps showed that more nearby quasars cluster like ‘normal’ galaxies,” explained Princeton University graduate student Yue Shen, who led the study. “But the clustering in our map is ten times stronger, the difference between a high contrast photograph and a washed out xerox.”

Quasars are glowing, ultraluminous concentrations of swirling gas falling into supermassive black holes at the centers of otherwise ordinary galaxies. Their great luminosities allow them to be seen at enormous distances, and since light travels at a finite speed, quasar maps provide a glimpse of structure when the universe was a small fraction of its current age.

“Quasars lie in galaxies, which lie in extended halos of invisible dark matter,” said Princeton University astronomer Michael Strauss, a member of Shen’s team. “In a typical galaxy, the dark matter outweighs the stars by ten to one.”

“We can’t observe the dark halos directly,” Strauss explained, “but we know from theoretical calculations how they should cluster with one another. By measuring the clustering of the quasars, we can infer the masses of the ‘halos’ in which they live.”

“We’ve shown that the brightest quasars, powered by the biggest black holes, lie in the most massive halos of the early universe, several trillion times the mass of the sun,” added Shen, “This is roughly what theories predict.”

The luminous distant quasars — powered by black holes up to a billion times the mass of the sun — are extremely rare, with average separations of 200 million light years or more. Before the SDSS, only a few hundred quasars had been discovered beyond 11 billion light years, the minimum distance of Shen’s sample,

“The SDSS made this possible by imaging a large area of sky to great depth, then following up the candidates to show which were true quasars,” said team member Gordon Richards of Drexel University. “Until you have a few thousand objects in your map, you can’t make this measurement.”

Because gravity pulls dark matter into denser structures over time, the clustering of dark matter in the early universe was much weaker than it is today. Richards explained the strong clustering of the brightest quasars as analogous to that of the highest mountain peaks on earth. “Most of them lie in the Himalayas, the Andes, the Rockies or the Alps.”

“There’s a whole low-altitude landscape of galaxies and dark matter,” said Richards, “but when you look for the brightest quasars you pick out just the snowcapped mountain ranges.”

The new measurements shed light on the early growth of supermassive black holes, according to theorist Avi Loeb of Harvard University, who is not a member of the SDSS-II team.

“The existence of bright quasars at early cosmic times is one of the unsolved mysteries of cosmology,” Loeb said. “How did black holes grow to a billion times the mass of the sun when the universe was only a tenth of its current age? The SDSS measurements will help us answer this question.”

The results are described in the paper “Clustering of High Redshift (Z > 2.9) Quasars from the Sloan Digital Sky Survey,” which was posted to the astro-ph web archive today. It has been accepted for publication in The Astronomical Journal.


Figure is available at

The illustration shows the distribution of dark matter, massive halos, and luminous quasars in a simulation of the early universe, shown 1.6 billion years after the Big Bang. Gray-colored filamentary structure shows the distribution of “invisible” dark matter. Small white circles mark concentrated “halos” of dark matter more massive than 3 trillion times the mass of the sun. Larger, blue circles mark the most massive halos, more than 7 trillion times of the sun, which host the most luminous quasars. The strong clustering of the quasars in the SDSS sample demonstrates that they reside in these rare, very massive halos.

The box shown is 360 million light years across.

CREDIT: Paul Bode and Yue Shen, Princeton University

A complete list of authors can be found at


The Sloan Digital Sky Survey-II ( is the most ambitious survey of the sky ever undertaken. With more than 300 astronomers and engineers in 25 institutions around the world, the SDSS-II is continuing to map one quarter of the entire sky, determining the position and brightness of hundreds of millions of celestial objects, including the measurement of distances to more than a million galaxies and quasars from the Apache Point Observatory in New Mexico.

In addition, the SEGUE (Sloan Extension for Galactic Understanding and Exploration) will undertake the mapping of the structure and stellar makeup of the Milky Way Galaxy. The new Supernova Survey will repeatedly scan a 300 square degree area to detect and measure supernovae and other variable objects.

Funding for SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagakusho, and the Max Planck Society.

The SDSS-II is managed by the Astrophysical Research Consortium for the Participating Institutions. The Participating Institutions are the American Museum of Natural History, Cambridge University, Case Western Reserve University, the University of Chicago, The Department of Energy’s Fermi National Accelerator Laboratory, the Institute for Advanced Study, the Japan Participation Group, the Johns Hopkins University, the Joint Institute for Nuclear Physics, the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist Group, Los Alamos National Laboratory, the Ohio State University, the Max-Planck-Institute for Astronomy, the Max-Planck-Institute for Astrophysics, New Mexico State University, University of Pittsburgh, University of Portsmouth, Astrophysical Institute Potsdam, Princeton University, the United States Naval Observatory and the University of Washington.


Yue Shen, Princeton University, 1-609-258-8057,

Michael Strauss, Princeton University, 1-609-258-3808,

David Weinberg, Scientific Spokesperson, The Sloan Digital Sky Survey, 1-614-292-6543,

Gary S. Ruderman, Public Information Officer, The Sloan Digital Sky Survey, 1-312-320-4794,