Johns Hopkins Gazette: April 22, 1996

On Research:  Galaxy Harbors Dark Matter

Emil Venere
Homewood News and Information

     Astronomers have discovered a galaxy about 17 million
light-years away that contains an unusually high concentration of
dark matter, offering a promising specimen with which to
investigate that unseeable stuff making up most of the universe. 

     When viewed with optical telescopes, the galaxy appears to
be a mundane, fuzzy dot. But when observed with a radio
telescope, a dramatic feature is revealed: an expansive disk, and
spiral arms of gas, is spreading out from the central visible

     "Normally, when you see spiral arms, you see stars in the
arms, but there is nothing there," said Gerhardt Meurer, a
postdoctoral fellow in the Department of Physics and Astronomy.

     No stars are mixed with the gas, which is called neutral
hydrogen, since it has not been ionized by radiation or shock
waves from exploding stars. Meurer and other astronomers 
observed the galaxy with the Australia Telescope Compact Array, a
collection of radio telescopes near Narrabri, Australia. 

     The galaxy, called NGC2915, is in the southern constellation
Chamaeleon, and it harbors an unusually high proportion of gas to
stars--about 2.7 times more neutral hydrogen than the combined
mass of its stars. In comparison, Earth's galaxy, the Milky Way,
has only about one-quarter as much gas as the mass of its stars.

     "You can say this galaxy is largely unevolved because it
still has so much gas compared to stars," Meurer said. 
The only part of the galaxy that is formed is its center. 

     The findings are detailed in a paper published this month in
The Astronomical Journal.     

     The astronomers discovered that at least 95 percent of
NGC2915's mass is in the form of dark matter, and the density of
dark matter in the galaxy's center is more than 10 times greater
than that typically found in normal spiral galaxies. The findings
are based on observations the astronomers made between May 1992
and March 1993. Because it is difficult to resolve small objects
with radio telescopes, they are not commonly used to observe
galaxies like NGC2915, which are called blue compact dwarf

     "In some ways it's like a first step, a sign to astronomers
to start paying attention to these blue compact dwarf galaxies to
try to understand dark matter," Meurer said.

     The existence of dark matter in the universe has been
inferred from a number of astronomical observations. According to
many theories and observations, the universe contains far more
matter than is directly observed using current technology.
Astronomers think that at least 90 percent of the mass in the
universe is derived from dark matter. The possible constituents
of the dark matter range from gases to stars that are too faint
to be detected by standard methods and exotic particles that do
not emit visible light.

     The dark matter's gravity in NGC2915 may act like a sort of
"glue" that concentrates the gas toward the center of the galaxy
so that stars can form, Meurer noted. 

     When stars die, they explode as supernovas, driving gases
outward with tremendous force. Something must keep the gas
together long enough for it to form new stars. 

     In normal galaxies the gravitational attraction of the stars
and gas is sufficient to hold the gas together under these
explosive actions. In NGC2915, dark matter provides the dominant
gravitational attraction, and the unusual concentration of dark
matter may result in a higher intensity of star formation than
normally seen in galaxies, Meurer said.

     Dark matter cannot be seen directly, but it can be detected
and measured through its gravitational force, which causes much
more rapid orbital motions of gas and stars than can be
attributed to the visible matter. Meurer's team found that the
neutral hydrogen gas in NGC2915 had the curious property of
mimicking the distribution of dark matter. This result confirmed
earlier findings that neutral hydrogen often has the uncanny
ability to trace dark matter's presence.

     The hydrogen gas also holds vital secrets about the
formation of stars and the evolution of galaxies, since stars are
created from gas. One reason for the galaxy's unusual makeup
could be its isolated place in the cosmos: normally galaxies
exist in groups or clusters, but NGC2915 is off by itself. The
gravitational interactions of neighboring galaxies would have
torn the fragile disk of neutral hydrogen apart, Meurer said.

     When viewed with optical telescopes, blue compact dwarf
galaxies appear much smaller than giant elliptical and spiral
galaxies, such as the Milky Way. The diameter of NGC2915's small
core of stars is only one-fifth the diameter of the Milky Way's
stellar spiral. It has only 11 percent of the Milky Way's mass,
and it is only 1.7 percent as bright, in the visible spectrum, as
the Milky Way. But the radio observations are striking because
they show NGC2915 to be a much larger galaxy, with a spiral disk
about 75 percent as large as the Milky Way's disk. The findings
mark the first time that a blue compact dwarf galaxy has been
used to calculate the mass distribution of dark matter.

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