Clustering in Universe Seen as Indicator of Galaxy Evolution

The spiral galaxy NGC 1087 is found in the constellation Cetus. (Image: SDSS Collaboration)

June 2, 2003—The discovery of differing ways that galaxies cluster in space has led researchers at the Sloan Digital Sky Survey (SDSS) to new insights into the evolution of galaxies and matter in the universe. The SDSS is a project funded by a large collaboration that includes the U.S. Department of Energy's Office of Science.

"The clustering of galaxies is directly related to the distribution of matter in the Universe today," explained principal investigator Tamas Budavari of Johns Hopkins University. "How matter is distributed today reflects conditions when the universe was less than a second old." Mapping the distribution of matter allows scientists to test theories for the origin of the Universe.

Now scientists with the SDSS have devised a method of obtaining not only a census of newly differentiated red and blue galaxies, but have begun to make critical findings related to the two populations. A key finding is that clustering properties do not appear to be continuous from one galaxy type to another as had been assumed.

The discovery enables progress in understanding the connection between galaxy properties, their environments, and their evolutionary histories. It could also help to unravel the mystery of the nature of dark matter.

Budavari presented the team's findings on May 26, 2003, at the American Astronomical Society meeting in Nashville, Tennessee.

SDSS investigators seek to understand the clustering of galaxies based on their types. Astronomers have known for years that galaxies of different evolutionary ages have different colors and shapes. Red elliptical galaxies today are composed of older, less active or inactive stars. Blue spiral galaxies are still in the star formation stage and are evolutionarily younger.

These two images show the density of red and blue galaxies in the same area of the sky. Since red galaxies cluster more strongly, their image (top panel) shows larger high density regions. Conversely, the higher density regions of the blue galaxies are smaller and more spread out.

After the Big Bang 14 billion years ago, primordial matter (mostly hydrogen and helium) congealed into galaxies. Pockets of matter (dark matter and the gas that forms the stars we see) within the universe collapsed under the force of gravity. Some of the earliest galaxies to form are what we see today as the red elliptical galaxies.

Over time the matter continues to cluster via gravity so that, in the universe we observe locally, ellipticals tend to reside in very dense regions (i.e., regions with a lot of galaxies that astronomers refer to as regions that are strongly clustered). Galaxies we see today as blue or spirals formed later than the ellipticals so the regions they occupy have had less time to accumulate matter and are less clustered.

The SDSS team also reported, as part of these new findings, is that while all galaxies were from smaller components, elliptical galaxies have a longer merging history than spiral galaxies and, that the two galaxy types clump in different ways.

Andrew Connolly, University of Pittsburgh

"As you go from red to blue you find they cluster differently," explained Andrew Connolly of the University of Pittsburgh. "This is surprising because I would have expected there to be a whole range of clustering signatures, not just two."

The SDSS team went beyond traditional use of spectroscopy to determine the three-dimensional clustering of galaxies, as spectroscopy can only be used for the brightest of objects, perhaps one percent of all the catalogued objects in a patch of surveyed sky.

Instead, the SDSS study carefully evaluated the colors of galaxies to determine their distances to create a three-dimensional map. In this way, SDSS scientists were able to create a uniform sample of millions of galaxies, each with quantified physical parameters such as their type and brightness. The result is an evaluation based on clustering properties of these distinct kinds of galaxies in different environments; what SDSS investigators considered a true apples-to-apples comparison.

The Sloan Digital Sky Survey observes the sky in stripes. Each of these eight long stripes contains millions of galaxies.

Budavari and Connolly have found that red galaxies tend to cluster more tightly and inferred that dark matter surrounding galaxies bunches similarly. Dark matter, a non-luminous or invisible matter that makes up as much as 27 percent of the mass of the universe, is known by its gravitational effects on visible celestial objects like galaxies.

The discoveries offer more questions to contemplate: Why do different types of galaxies cluster differently? Does differentiated clustering relate to dark-matter contents? Or does it relate to its evolutionary history? Are these connected?

Other SDSS lead investigators were Alex Szalay, also of Johns Hopkins; Istvan Szapudi of the University of Hawaii; Istvan Csabai of Eotvos University, Budapest, Hungary; and Ryan Scranton, also of the University of Pittsburgh.

The SDSS is the most ambitious astronomical survey ever undertaken. With more than 200 astronomers at 13 institutions around the world, the SDSS will map in detail one-quarter of the entire sky, determining the positions and absolute brightness of more than 100 million celestial objects. It will also measure the spectrographic distances to more than a million galaxies and quasars. The SDSS telescopes are located at Apache Point Observatory in New Mexico and operated by the Astrophysical Research Consortium.

The Sloan Digital Sky Survey will observe half of the northern hemisphere. This figure illustrates the sky coverage of Data Release 1, the current status of the survey.

The SDSS findings on galaxy clustering were culled from statistical analysis of tens of millions of galaxies. The first public data release from the SDSS, called DR1—or Data Release 1, contains about 15 million galaxies. Prior 3D sky mappings were able to survey just a few hundred thousand galaxies.

Even further, the SDSS was conducted with five color bands using charge coupled devices where earlier astronomical surveys were done in just one color, most using photographic plate images.

"Without the five-band photometry these findings wouldn't have been possible," said Connolly.

SDSS provides a uniquely powerful 3D map to make such studies: large in volume and numbers of galaxies as well as detailed and accurate parameters for each galaxy with uniform coverage.

"We're beginning to study these structures at a level of accuracy that we've never been able to do before. We can see in exquisite detail how the properties of galaxies change and really map out how they change in a great amount of detail," Budavari said.

Other SDSS collaborating institutions in this discovery are Princeton University, Princeton, New Jersey; Apache Point Observatory, Sunspot, New Mexico; Steward Observatory, Tucson, Arizona; The University of Chicago; Fermi National Accelerator Laboratory, Batavia, Illinois; and the Institute for Cosmic Ray Research, University of Tokyo.—by Gary Ruderman

Media contact: Gary Ruderman, SSDS Public Information Officer, (312) 320-4794, mailto:ssdspio@aol.com
Technical contacts: Tamas Budavari, Johns Hopkins University, (410) 516-0643, mailto:budavari@jhu.edu; Andrew Connolly, University of Pittsburgh, (412) 624-1345, mailto:ajc@tiamat.phyast.pitt.edu

Related Links

SSDS Data Release 1

"Sloan Digital Sky Survey Probes Dark Matter Theory," Gary Ruderman, SDSS, March 21, 2003.

Tamas Budvari, Johns Hopkins University

Andrew Connolly, University of Pittsburgh Department of Physics and Astronomy

Apache Point Observatory (site of SDSS telescopes)

Sloan Digital Sky Survey at Johns Hopkins University

"Most Distant Object Every Observed," DOE Office of Science's Decades of Discovery