Two 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

Astrophysicists 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.
Dalal 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.

The 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

“The 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.”
One 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.