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May 20, 2002
Neal Dalal, Center for Astrophysics
and Space Science, UCSD,
Kochanek, Harvard-Smithsonian Center for Astrophysics,
of the quasar MG 0414+534, showing multiple images due to
EVIDENCE FOR DARK DWARF GALAXIES
scientists have found evidence that galaxies are surrounded by halos
containing hundreds of invisible dwarf galaxies. Their discovery, described in
a paper in the June 10 issue of The Astrophysical Journal, provides
strong support for the theory that most of the matter in the universe is in
the form of some undetected type of slowly moving particles called cold dark
Neal Dalal of the University of California, San Diego, and Christopher
Kochanek of the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA,
based their conclusion on an analysis of the gravitational lensing of light
from distant galaxies by intervening galaxies. According to Einstein’s
theory of gravity, large concentrations of matter such as galaxies can warp
the surrounding space and bend the light from distant galaxies in much the
same way that a glass lens can bend light.
One consequence of gravitational lensing is that the image of a single galaxy can be split into two or more images. The number and appearance of these multiple images depend on the distribution of mass in the lensing galaxies. In particular, if a lensing galaxy is surrounded by many smaller dwarf galaxies, the brightness of one of the lensed images could be significantly enhanced if it were aligned with one of the dwarf galaxies.
showing how multiple images are produced by gravitational
and Kochanek performed a statistical analysis of 7 different lensing galaxies
which had each split the light of a background galaxy into 4 images of varying
brightness. They found that about 2 percent of the mass of the lensing
galaxies must be in the form of dwarf galaxies in order to explain the
observed brightness variations among the multiple images of the background
galaxies. Their study could help vindicate a model for the formation of
galaxies in the eons after the Big Bang. A growing body of evidence
indicates that from 80 to 90 percent of the matter in the universe is in the
form of an as yet unknown type of elementary particle that contributes to
gravity through its mass but otherwise interacts weakly with normal matter
composed of protons and neutrons.
currently favored form of dark matter is cold dark matter, so-called because
the particles are assumed to move slowly, making it easier for gravity to pull
them together to form galaxies. A firm prediction of this model is
that large galaxies such as our Milky Way Galaxy should have numerous small
satellite galaxies around them. The failure of astronomers to find the
predicted swarms of dwarf galaxies around large galaxies has led a number of
scientists to call for the abandonment of the cold dark matter model.
lack of observed satellite galaxies around large galaxies has been a major
point in the prosecution of the case against cold dark matter,” said Dalal.
“Our result can be regarded as a major vindication of the model.”
puzzle remains as to why the dark dwarf galaxies contain few or no stars, if
10 to 20 percent of their mass is in the form of normal matter. “It’s
difficult to hide that much material,” said Dalal. “Perhaps most of
the gas was stripped from the dwarfs when the galaxy was formed.”
Their research was supported by the National Aeronautics and Space Administration, U.S. Department of Energy, Smithsonian Institution and the ARCS Foundation.
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