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New evidence from NASA’s Chandra X-ray Observatory challenges an
alternative theory of gravity that eliminates the need for dark matter.
The observation also narrows the field for competing forms of dark
matter, the elusive material thought to be the dominant form of matter
in the universe.
An observation of the galaxy NGC 720 shows it is enveloped in a slightly
flattened, or ellipsoidal cloud of hot gas that has an orientation
different from that of the optical image of the galaxy. The flattening
is too large to be explained by theories in which stars and gas are
assumed to contain most of the mass in the galaxy.
“The shape and orientation of the hot gas cloud require it to be
confined by an egg-shaped dark matter halo,” said David Buote of the
University of California, Irvine, and lead author of a report on this
research in the September 20, 2002 issue of The Astrophysical Journal.
“This means that dark matter is not just an illusion due to a
shortcoming of the standard theory of gravity — it is real.”
According to the generally accepted standard theory of gravity, the hot
X-ray cloud would need an additional source of gravity — a halo of dark
matter — to keep the hot gas from expanding away. The mass of dark
matter required would be about five to ten times the mass of the stars
in the galaxy. If the dark matter tracked the optical light from the
stars in the galaxy, the hot X-ray cloud would be more round than it
is. The flattened shape of the hot gas cloud requires a flattened dark
matter halo.
An alternative theory of gravity called MOND, for Modified Newtonian
Dynamics, was proposed in 1983 by Mordecai Milgrom of the Weizmann
Institute in Israel, and has remained viable over the years. MOND does
away with the need for dark matter by modifying the theory where the
acceleration produced by gravity is very small, such as the outskirts of
galaxies. However, MOND cannot explain the Chandra observation of NGC
720. This is apparently the first dynamical evidence that has
successfully distinguished dark matter from MOND.
The researchers also found that the Chandra data fit predictions of the
cold dark matter theories, according to which dark matter consists of
slowly moving particles, which interact with each other and “normal”
matter only through gravity. Other forms of dark matter, such as
self-interacting dark matter, and cold molecular dark matter, are not
consistent with the observation in that they require a dark matter halo
that is too round or too flat, respectively.
“Chandra’s ability to precisely identify and locate the point-like
sources contaminating the diffuse emission in the X-ray image was
absolutely essential,” said Buote. “Only then could we make accurate
measurements of the shape and orientation of the X-ray image contours.”
The conclusion from the Chandra data that NGC 720 possesses a dark
matter halo assumes that the hot gas cloud has not been unduly disturbed
by collisions or mergers with other galaxies in the last 100 million
years. The lack of evidence of such activity indicates that this
assumption is valid.
Chandra observed NGC 720, which is about 80 million light years from
Earth, for 11 hours with the Advanced CCD Imaging Spectrometer (ACIS).
Other members of the team include Tesla Jeltema and Claude Canizares of
Massachusetts Institute of Technology (MIT) in Cambridge, and Gordon
Garmire of Pennsylvania State University in University Park. Penn
State and MIT developed the instrument for NASA.
NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the
Chandra program, and TRW, Inc., Redondo Beach, Calif., is the prime
contractor for the spacecraft. The Smithsonian’s Chandra X-ray Center
controls science and flight operations from Cambridge, Mass.
Images and additional information associated with this release are
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