Clustering in Universe Seen as Indicator
of Galaxy Evolution
Researchers at the Sloan Digital Sky
Survey have discovered surprising new information about
how galaxies cluster in space, leading to new information
about evolutions of galaxies and matter in the universe.
These findings were presented at the American Astronomical
Society meeting in Nashville, Tennessee, on May 26,
2003.
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The spiral galaxy NGC 1087
is found in the constellation Cetus. (Image: SDSS
Collaboration) |
June 2, 2003The
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.
Galaxy Densities Differentiated
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.
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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.
SDSS Large-Scale Undertaking
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 DR1or 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
Funding:
Funding for the Sloan Digital Sky Survey
is provided by the U.S. Department of Energy's
Office of
Science's through its High
Energy and Nuclear Physics program, the Alfred
P. Sloan Foundation, the National
Aeronautics and Space Administration, the
National Science
Foundation, the Japanese
Monbukagakusho, and the Max
Planck Society.
The Sloan
Digital Sky Survey is managed by
the Astrophysical Research Consortium for the
Participating Institutions. The Participating
Institutions are The University of Chicago, DOE's
Fermi National Accelerator Laboratory, the Institute
for Advanced Study, the Japan Participation Group,
The Johns Hopkins University, DOE's Los Alamos
National Laboratory, the Max-Planck-Institute
for Astronomy (MPIA), the Max-Planck-Institute
for Astrophysics, New Mexico State University,
University of Pittsburgh, Princeton University,
the United States Naval Observatory, and the University
of Washington.
The SDSS will map in detail
one-quarter of the entire sky, determining the
positions and absolute brightness of 100 million
celestial objects. It will also measure the distances
to more than a million galaxies and quasars. The
Astrophysical Research Consortium operates Apache
Point Observatory, site of the SDSS telescopes.
Author: Gary Ruderman
is the public
information officer for the Sloan Digital Sky
Survey. For more news, see the current SDSS news
releases.
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